Incorporating Data-Driven
Safety Analysis
in Traffic Impact Analyses:
A How-To Guide
2
Introductio n
Background
Traditional crash and roadway analysis methods mostly rely on subjective or limited quantitative measures of safety
performance. This dependence makes it challenging to calculate safety impacts alongside other criteria when planning
projects. Data-Driven Safety Analysis (DDSA) employs newer, evidence-based models that provide State and local
agencies with the means to quantify safety impacts similar to the way they do other impacts such as environmental
eects, trac operations, and pavement condition. DDSA provides reliable estimates of an existing or proposed
roadway’s current and future safety performance and helps agencies make more informed decisions, better target
investments, and reduce crashes occurring on their roadways. This guide demonstrates how transportation agencies
and consultants can incorporate DDSA into routine trac impact analyses (TIA).
TIAs are engineering studies that estimate the impacts of a proposed trac generator (e.g., new residential or commercial
development) on the transportation system. Traditionally, TIAs focused on the capacity and operational aspects of
increased trac volumes, assuming that improvements to address operational impacts would also provide safety
benets. However, independent safety analysis was not usually done, resulting in safety impacts being overlooked, and
opportunities for safety enhancements being missed. This was often due to the diculty of quantifying future safety
performances and their impacts. Additionally, there were challenges associated with the assumed level of eort (e.g.,
cost) and assumed experience required to conduct a safety analysis. With the advent of these new models and tools,
practitioners can now more readily integrate DDSA into the TIA process.
Credit: Fotosearch
3
Benets of Using DDSA in Trafc Impacts Analyses
Incorporating safety into TIAs will help highlight safety issues during the project development phase before any
construction is completed, thereby enabling the development of safer roadways. Although crash reduction is a primary
goal for the transportation agency and the public, safety analysis is more about nding reasonable solutions that
mitigate the impacts of a proposed development. From the land developer’s perspective, crashes cause delays and can
make it more dicult to get customers or residents into the development, so any eort that reduces crashes is a benet
to the developer. Furthermore, safety driven improvements will help agencies be more receptive to the development
overall and therefore may help facilitate and expedite the necessary approvals to move the project forward. From the
transportation agency’s perspective, incorporating safety into a TIA can help the agency achieve targeted benchmarks
for a reduction in crashes and fatalities, which are often part of initiatives such as Vision Zero and Toward Zero Deaths.
Additionally, implementing safety as part of the TIA can provide the citizens and roadway users a more comfortable
and safer environment as they commute home to loved ones.
Safety Tools Used
There are a variety of methods that transportation agencies and consultants can use to integrate DDSA into TIAs. This
guide provides a high-level overview of how to conduct safety analyses using readily available tools and data. AASHTO,
FHWA, and the private sector have developed several tools for conducting robust analyses, and more information on
these tools can be found at Data-Driven Safety Analysis Resources - DDSA Toolbox. In addition, some state and local
agencies have their own customized tools. Therefore, the analyst conducting the safety analysis should check with the
agency about tool and data availability.
Credit: Fotosearch
4
Overview of DDSA in Trafc Impact Analyses
Figure 1 shows the integration of DDSA into the typical TIA process, with the DDSA related elements shown in
bold lettering. This example represents a typical TIA with a proposal for future development. At the beginning of the
TIA process, the applicants for the project contact localities and begin their preliminary due diligence and identication
of the study area. In doing so, the analyst develops a site plan and an ingress/egress conceptual plan. Next, the analyst
will begin evaluating existing conditions by thoroughly reviewing physical characteristics, collecting trac data, as well
as evaluating existing operations and determining the level of service. Additionally, the analyst will begin to identify
and obtain safety data and evaluate pedestrian and bicycle accommodations. The analyst will use obtained safety
data and will, in turn, develop visual aids from the data to identify safety issues. Next, the analyst will evaluate existing
intersection operations and capacity and project volumes into the future conditions. These projected volumes will be
used to evaluate the future no-build alternative and all build alternatives. Once the analyst has an understanding of
the existing and future no-build conditions, preliminary alternatives are developed and tested from an operations and
safety perspective to understand the implications of the new development. The analyst will not only utilize the results
of these analyses to compare the alternatives, but will typically nalize the comparison by conducting a benet-cost
analysis. Based upon these results and the build conditions, the analyst would make recommendations to improve
both operational and safety impacts of the proposed development.
Figure 1: Opportunities to Integrate Safety in the Typical Trafc Impact Assessment Process
Proposed
Development
Existing
Conditions
Projected
Growth
Traffic
Analysis
• Identify the Study Area
• Develop the Site Plan
• Develop Ingress/Egress Conceptual Plan
• On-Site/Off-Site Land Use Development
• Document Physical Characteristics
• Collect Traffic Volumes and Non-motorized Data
• Identify and Obtain Safety Data
• Evaluate Pedestrian and Bicycle Accommodations
• Summarize Data and Identify Safety Issues
• Perform Capacity/Queueing Analyses for Existing Conditions
• Background Traffic Forecasting
• On-Site and Off-Site Development Traffic Forecasting
• Build and No-Build Conditions Total Traffic
Black: typical traffic impact assessment step
Bold: opportunity to integrate safety
• Develop Preliminary Alternatives
• Evaluate Site Access
• Perform Capacity/Queueing Analysis
• Traffic Control/Geometric Consideration
• Perform Safety Analysis of Each Preliminary Alternative
• Compare Alternatives and Make Recommendations
5
Application of DDSA to Trafc Impacts Analyses
The following sections represent DDSA action items, within the existing TIA process, that will implement safety into
TIAs. These nested action items, as outlined in bold lettering, will be conducted in the order as shown in Figure 1.
Proposed Development
Action Item: Develop an Ingress/Egress Conceptual Plan
The rst step is to develop an understanding of the existing ingress and egress locations for the existing access way.
This requires either observing the conditions in the eld or discussing adjacent development site access with local
jurisdictions. Conceptualizing these entrance and access points will help gain a preliminary understanding of the
existing conict points or issues related to the intersection and can help provide evidence supporting crash data that
may be unclear. Additionally, it is important to consider this conceptual plan as part of the analysis further in the TIA.
Relying on existing entrances or intersection should not be the assumed path for the development; however, it should
be carefully considered when proposing new driveways. The analyst should consider the following when locating
proposed access points:
• The number of access points should be limited to minimize trac conicts.
• The ingress/egress conceptual plan should align opposing access points where possible.
• The location of access points should maintain adequate spacing between adjacent streets and driveway intersections.
• The potential for joint and/or cross access between adjacent properties.
It is also important to note that, although the development of an ingress/egress conceptual plan occurs early in the
TIA process, it is not a xed project element and should be revised if safety and operational issues are detected after
analysis.
Existing Conditions
Action Item: Identify and Obtain Crash Data
The analyst should next obtain safety data to review existing conditions. Typically, analysts will use three to ve years
of crash data, and this should be discussed with the road owner before beginning. To conduct a safety analysis, it is
crucial to review and understand the crash narratives. While the level of detail within crash records varies depending on
the reporting agency and ocer, at the very minimum, obtaining the when, where, and what type of crash are helpful
in conducting the safety analysis. Additionally, details of the crash severity and causation are also useful. Analysts can
typically obtain crash data from the local transportation agency or the State DOT. If the data is not available from either
of those sources, the analyst can contact local law enforcement ocials to request copies of the crash reports or a
database of consolidated crash data.
Action Item: Evaluate Pedestrian and Bicycle Accommodations
When proposing any development, it is essential to consider how a project impacts both motorized and non-motorized
users. Little or no actual crash data that reveals trends involving vulnerable road users means the analyst must consider
the nature of the development and its proximity to other land uses where walking and biking may occur. Evaluating
other considerations, such as bus stops and pedestrian facilities, is also important to gain an understanding of the
types of pedestrian volumes and uses of adjacent area pedestrian facilities. This will provide the analyst with insight
as to what additional facilities should be implemented as part of the development that will reduce existing crashes or
mitigate potential future crashes involving non-motorized users.
6
Action Item: Summarize Data and Identify Safety Issues
With the data collected, the next step is to present the data visually to understand problematic locations and recurring
crash types. The analyst could accomplish this step in several ways, including plotting crashes on a pin map, developing
a collision diagram, or using online geographic information systems (GIS) to pinpoint crash locations. Regardless of the
method chosen, summarizing the data will support the identication and analysis of safety issues.
One approach includes developing collision diagrams using aerial imagery and placing symbols at the crash locations
to represent each crash. Some states use analytical tools to assist with obtaining, processing, and visualizing crash data
in order to develop the crash diagrams. Figure 2 shows an example of a collision diagram. In this image, the dierent
symbols represent dierent crash types (e.g., rear-end, angle), which provide the analyst an understanding of common
safety issues in the area. For example, Figure 2 shows rear-end crashes near an access point to residential apartment
buildings, and this information may warrant a closer examination of the roadway at and around this area to determine
potential contributing factors and the need to consider safety improvements.
Figure 2: Example Collision Diagram
After completion of the crash data visualization, the analyst will summarize the existing safety issues. This portion of
the analysis will include crash pattern summaries that typically are presented in tabular or graphical format. Figure 3
and Figure 4 illustrate examples of graphs that show crashes by type and by time-of-day at a specied location. These
graphs provide an easy and straightforward way to visualize the existing crash conditions.
Analysts can also identify existing safety issues by performing a site visit, or a more in-depth road safety audit (RSA).
A RSA is a formal safety performance evaluation that qualitatively estimates and reports on the potential road safety
issues. RSAs can also identify opportunities for improvement, which can aid the analyst in developing alternatives to
evaluate later in the TIA process.
Credit: Toxcel
Image provided by Google 2019
7
Figure 3: Crashes by Type
Figure 4: Crashes by Time of Day
2
4
3
1
0
0.5
1
Rear End Angle Non-Collision Fixed Object -
Off Road
Types of Crashes (2012 - 2017)
Number of Crashes
0
1
1.5
2
2.5
3
3.5
1.5
2
2.5
3
3.5
4
4.5
1
0
1
2
0
3
2 2
0
0.5
1
1.5
2
2.5
3
3.5
12 AM to
3 AM
3 AM to
6 AM
6 AM to
9 AM
9 AM to
12 PM
12 PM to
3 PM
3 PM to
6 PM
6 PM to
9 PM
9 PM to
12 AM
Time of Day (2012 - 2017)
Number of Crashes
8
Analysis
Action Item: Develop Preliminary Alternatives
Typically, TIAs will evaluate existing conditions such as volume data, geometric features, and congurations and will
use this data to prepare preliminary alternatives for roadways and intersections impacted by the development. The
alternatives should consider both operational and safety performance as a result of the development. The issues
identied in the crash data visualization action item above, in combination with the existing operational factors, can
help the analyst prepare ne-tuned alternatives that improve intersection performance from both an operational and
safety standpoint. For example, an intersection approach could currently be experiencing a large number of rear-end
crashes in a shared left-thru lane. This approach also experiences severe delay due to the large number of drivers waiting
to make the left-turn movement. An alternative that recommends the installation of an additional left-turn lane could
not only reduce delay, but also reduce the number of rear-end crashes at the intersection. Although the preliminary
alternatives should be developed based on the issues present at the intersection or roadway, safety and capacity/
operations analysis of these alternatives will be evaluated for the future conditions in future steps of the TIA process.
Action Item: Perform Safety Analysis of Each Preliminary Alternative
Following the analyst’s evaluation of the future build and no-build scenarios from an operational perspective, safety
analyses will then be conducted to determine the safety performance of each scenario. One method for evaluating
safety impacts is the application of Crash Modication Factors (CMFs), which is the value assigned to a certain specic
countermeasure to estimate the number of expected crashes following implementation. Another method involves
conducting safety analysis using the HSM predictive method. Depending on the complexity of the TIA and the data
available, some methodologies would be more benecial than others, and some may be used concurrently. TIAs with
detailed design concepts and available data should be evaluated using the HSM Part C predictive method. The HSM
predictive method uses safety performance functions to estimate future crash frequencies using various parameters
(e.g., AADT, roadway congurations, and existing crash data). The following section outlines these methods.
HSM Part C Predictive Method
The preferred method to evaluate safety benets is by conducting a predictive analysis. The HSM provides analysts with
a predictive method for estimating crash frequency at specic sites. This method, described in Part C of the HSM, uses
safety performance functions (SPFs) – along with CMFs and calibration factors – to estimate the crash frequency of sites
based on various characteristics (e.g., roadway characteristics, roadway/intersection geometry, area characteristics).
Using the predictive method provides an analyst with an eective and data-driven approach to determining future
crash frequency based on the conditions of the roadway.
While the HSM provides equations for analysts to manually estimate crash frequency based on characteristics of the
roadways, intersections, or freeways, there are many tools that implement HSM’s predictive method in an automated
format. This includes several Microsoft Excel-based spreadsheets as well as software developed under NCHRP projects
or FHWA.
1
Specically, FHWA developed two tools to conduct predictive method analysis at the site-specic, project
level: The Safety Performance of Intersection Control Evaluation (SPICE) and the Interactive Highway Safety Design
Model (IHSDM). Both tools are user-friendly applications which help predict safety performance (crash frequencies and
severities) for a site or set of sites (e.g., a section of highway, an intersection) and can be used in complex or relatively
simple safety analyses. Each of these tools is described below. It is critical to note that, while both tools provide users
with default SPFs and calibration factors, many states have also developed their own for use with HSM models. These
local values should be used when appropriate.
1 American Association of State Highway and Transportation Ocials. (2019). Highway Safety Manual - Tool Descriptions. Retrieved from http://
www.highwaysafetymanual.org/Pages/tools_sub.aspx#4
9
SPICE Tool
The SPICE tool is a Microsoft Excel-based macro workbook that can perform the predictive safety analysis of various
types of intersections with readily or limited available data-inputs. With this tool, analysts can assess safety performance
while considering performance metrics such as quality of operational service, construction maintenance costs, project
context, and other factors. This tool prompts users for basic inputs, automating many of the decisions required for
selecting the appropriate SPF or CMF to apply. Based on the input parameters, the tool outputs the predicted or
expected crash frequency and crash severity for each alternative, allowing for easy comparison.
IHSDM
IHSDM is a standalone desktop application which provides estimates of expected safety and operational performance
based on user-provided highway and intersection designs. This tool prompts users for various data inputs (e.g., number
and widths of lanes, intersection control, trac volume, and historical crash data) and can output predicted or expected
crash frequencies.
HSM Part D CMF Application
One method to quickly estimate the safety impacts of changes to transportation infrastructure is the use of CMFs.
A useful source for CMFs is FHWA’s CMF Clearinghouse,
2
which also includes information on using CMFs. The CMF
method is also documented in Part D of the Highway Safety Manual.
Analysts can use Equation 1 to estimate the number of crashes after a roadway modication is made.
Where:
Cestimate= the estimated crash frequency (crashes per year) after the roadway modication
Cexisting= the crash frequency before the roadway modication (crashes per year)
CMF= the crash modication factor
Equation 1 can also be used with multiple CMFs, as shown below.
Where:
2 Federal Highway Administration. (2018). Crash Modication Factors Clearinghouse. Retrieved from http://www.cmfclearinghouse.org/
C
estimate
= C
existing
* CMF
C
estimate
= C
existing
* CMF
1
* CMF
2
* CMF
n
(1)
CMF
n
= the crash modication factor associated with countermeasure n
10
Action Item: Compare Alternatives and Make a Recommendation
Traditionally, recommendations for roadways or intersections were made based on the operational performance and
costs of improvements, and often failed to consider safety impacts. Now, recommendations can be made based on
operational, safety, and cost considerations.
The analysts can select the most optimal alternative by performing a benet-cost analysis. This analysis compares
all of the benets (e.g., crash reduction) associated with countermeasures as compared to the cost of implementing
the countermeasure, in the form of a ratio (benet/cost). These analyses allow the developer and agency to quantify
impacts in a monetary form as a way of easily comparing the cost and safety benets oered in each alternative.
Additionally, analysts can also conduct an incremental benet-cost analysis, which produces a ranking of dierent
projects to determine which project is the best economic investment. Two sources that are currently available to assist
with the implementation of benet-cost analyses are FHWA’s Highway Safety Benet Cost Analysis Guide and tool.
3
Analysts can also perform a high-level comparison of alternatives using the crash prediction values generated by the
SPICE tool. Depending on the analysis selected, the predicted total and fatal-injury crash frequencies are displayed for
the opening year, design year, and the total project life cycle. The analyst can then feed these values into the BCA tool
to compare the alternatives Analysts can also perform a similar comparison using the Economic Analysis module built
into the IHSDM. Both of these tools can help analysts compare the alternatives with respect to benets and costs.
Analysts conducting these analyses will be faced with dicult decisions as they make recommendations to the agency,
and thus should exercise engineering judgment in making recommendations. For example, it can be dicult for
analysts to recommend one alternative that has slightly better safety outputs with considerably higher construction
costs versus a more cost-eective alternative that has fewer safety benets (i.e., crash reduction). While states and
agencies may have varying policies on how to evaluate and properly select alternatives, they should serve to evaluate
whether the proposed improvements satisfy the project goals, meet the needs of motorized and non-motorized users,
and whether they benet both the community and stakeholders.
Other Considerations
In addition to the crash analysis described above, the analyst should consider other safety-related elements when
completing a TIA.
4
Analysts should review the proposed plans to determine if:
• The necessary access points are available for the project.
• The existing and proposed access points are suciently spaced to reduce the risk of conicts.
• Trac control or geometric design is needed to restrict left turns.
• There are opportunities to consolidate the number of access points (e.g., shared driveway).
• Sight distance is adequate for all new and modied facilities.
• Adequate pedestrian and bicyclist facilities exist in the proposed plan.
• Considerations for commercial vehicle trac are included.
• Adjacent transit points exist along the adjacent roadway(s) and how the proposed development could impact the use of
these transit points.
3 Federal Highway Administration. (2019). Highway Safety Improvement Program (HSIP) – Planning. Retrieved from https://safety.fhwa.dot.gov/
hsip/planning.cfm
4 McRae, J., Bloomberg, L., & Muldoon, D. (2006). Best Practices for Trac Impact Studies (Final Report No. FHWA OR-RD-06-15). Salem, OR.
Retrieved from https://www.oregon.gov/ODOT/Programs/ResearchDocuments/BestPracticesforTrac.pdf
11
Overcoming Potential Challenges
This section discusses some of the possible challenges faced when integrating safety into TIAs and how to overcome
those challenges.
Operational Analysis vs. Safety Analysis
While operational and safety analyses aim to support each other, many TIAs prioritize operational benets, with safety
benets being secondary. Often, safety considerations are only included in a TIA when they are convenient or are
required by the local agency, and even then, these considerations are not truly integrated into the design process. From
the operational perspective, design elements are typically evaluated using periods of peak demand. It is important to
realize that what is operationally better for peak-period operations is not necessarily what is best for safety under the full
range of conditions. Integrating DDSA into a TIA can help the analyst understand how dierent proposed designs will
function from separate operational and safety standpoints, leading to a recommendation that then jointly considers
both of these inputs.
Difculty Comparing Proposed Improvements
Dierent roadway improvements will aect safety dierently. For example, it may seem dicult to compare an
improvement that reduces a large number of minor crashes with one that reduces a small number of more severe
crashes, but researchers and analysts have developed costs that help do just that.
5
Crash costs, often dened by the
crash severity and crash type, allow analysts to summarize the safety impacts of roadway improvements in a way that
allows simple comparisons. Further, dening safety impacts as a monetary value not only allows the analyst to compare
safety impacts to each other, it allows the analyst to compare these impacts to roadway improvement cost and show
the ratio of costs to benets for the roadway improvement.
The Credibility of the Safety Analyses
Data-driven safety analysis builds on decades of research and collaboration by AASHTO, TRB, and FHWA. In 2016, FHWA
published a series of ve informational guides on the Reliability of Safety Management Methods, which demonstrated
the value of the more reliable (predictive) methods highlighted in this how-to guide over traditional methods. These
methods allow the safety analysis to have a quantitative foundation, which allows the results of these analyses to be just
as compelling as the results of operational analyses. Additionally, to further improve the condence in safety analyses
performed using CMFs, the CMF Clearinghouse provides a quality rating of each CMF to help analysts select those
CMFs that have been developed through the most thorough analyses. It is recommended to check with the state and
local transportation agencies about their guidelines for the application of CMFs in a safety analysis; some agencies
provide guidelines on the minimum required quality level for CMFs or have a state-preferred CMF list.
Level of Effort Required to Complete a Safety Analysis
One reason transportation agencies and consultants often leave safety analyses out of TIAs is that they assume that
completing such analyses requires a signicant level of eort. However, as discussed in this guide, safety analyses can
be completed without expending a substantial level of eort and will add value to the resulting TIA. With the use of
GIS and online mapping tools, an analyst can review locations for existing conditions of an intersection or roadway,
and supplement this with feedback from local authorities about their knowledge of existing operational and safety
conditions. Additionally, FHWA also makes other tools available (e.g., SPICE, IHSDM) to facilitate a predictive safety
analysis. FHWA has developed these tools to use data-driven procedures supported by existing transportation safety
research. Using simple and available resources, analysts can conduct safety analyses that will provide insight for
recommendations that will benet the site from both operational and safety considerations.
5 Harmon, T., Bahar, G., & Gross, F. (2018). Crash Costs for Highway Safety Analysis (Final Report No. FHWA-SA-17-071). Washington, DC. Retrieved
from https://safety.fhwa.dot.gov/hsip/docs/fhwasa17071.pdf
12
Example: Trafc Impact Analysis (TIA) with Data-Driven Safety
Analysis Integration
The following section provides an example of how to integrate DDSA into the TIA procedure. The TIA example follows
a proposed new development (a pharmacy) on the corner of a signalized intersection within a moderately developed
area. The example describes the typical process for a TIA, but also includes the DDSA action items as part of the
procedure.
Figure 5: Proposed Development Location
Credit: Toxcel
Image provided by Google Maps 2019
13
Project Overview
Company X is proposing to develop a pharmacy with a drive-through on the northwest corner of Smith Road and Main
Street, shown in Figure 5. The area proposed for development is currently vacant and is primarily bound to the northeast
and south by commercial developments, with residential dwellings located to the west. The proposed development
plan consists of a 12,000 square foot development which includes a drive-through. The site can be accessed via the
west leg of the existing trac signal of Main Street at Smith Road. The development is being proposed across the street
from a large existing residential development, and currently the intersection has no pedestrian facilities. The project is
expected to be completed and opened by the year 2020.
Existing Conditions and Projected Growth
Site Development Trafc Forecasting
The analyst performed trip generation calculations for the proposed development using the Institute of Transportation
Engineer’s (ITE) Trip Generation report, 9th Edition. The analysis used ITE Land Use Code (LUC) 881: Pharmacy/Drugstore
with Drive-Through Window.
Based on the trip generation evaluation, the proposed development is expected to generate 46 gross new AM peak
hour trips (24 entering trips, 22 exiting trips) and 123 gross new PM peak hour trips (62 entering trips, 61 exiting trips).
A pass-by rate of 49 percent (49.0%) was applied based on ITE Trip Generation Handbook, generating 23 net new AM
peak hour trips (12 entering trips, 11 exiting trips) and 63 net new PM peak hour trips (32 entering trips, 31 exiting trips).
Table 1 indicates the breakdown of the trip generation.
Table 1: Trip Generation Calculations
Trip Generation Calculation for AM and PM peak hour trips
Gross Trips Pass-By Capture Net New Trips
Pharmacy with Drive-Through 46 (24 in/22 out)
49.0%
23 (12 in/11 out)
123 (62 in/61 out) 63 (32 in/31 out)
AM peak hour; PM peak hour
Credit: Fotosearch
Proposed Development
14
Volume Forecasting
The analyst collected existing condition turning movement counts for the intersection on a typical weekday in
15-minute intervals. Based on the existing trac volume data, the PM peak hour volumes were used for the analysis,
as the development is expected to generate higher volumes during the PM peak hour. Trac growth was projected
to 2020, the proposed build-out date, by applying a historic growth rate to existing 2018 trac volumes. The analyst
determined this growth rate based on historical growth trends along the Main Street corridor. The three sets of data
used for the analysis were:
o Existing Conditions (2018 volumes no-build)
o Future Background Conditions (2020 volumes no-build)
o Future Total Conditions (2020 volumes build)
Smith Road
Main Street
Existing
Development
60% (40%)
40% (60%)
100% (100%)
60%
40%
100%
(100%)
(-40%)
40% (40%)
60% (60%)
(-60%)
Existing
Development
Proposed
Pharmacy
LEGEND
Study Roadway
Study Intersection
Entering Distribution
Entering Pass-By Distribution
Existing Distribution
Existing Pass-By Distribution
20%
(80%)
20%
(80%)
Figure 6: Trip and Pass by Distribution
15
The analyst generated future total trac by taking the Future Background conditions (2020 volumes no-build) and
adding in the project trac (i.e., net new trips + pass-by trips) to the intersection.
Figure 6 and Figure 7 illustrate the expected project distribution, pass-by distribution, and driveway volumes, based
on the proposed development. For purposes of this trac impact analysis example, the PM peak hour will be the only
peak evaluated based on net new trips generated and the existing turning movement counts.
Figure 7: Trip and Pass by Assignment
Smith Road
Main Street
Existing
Development
19 (12)
12 (18)
32 (30)
60%
40%
31 (30)
(-12)
13 (12)
19 (18)
(-18)
Existing
Development
Proposed
Pharmacy
LEGEND
Study Roadway
Study Intersection
PM Net New Trips
PM Pass-By Trips
20
(80)
16
Pedestrian Volumes
Along with vehicular volumes, the analyst also collected pedestrian volumes for the intersection on a typical weekday
in 15-minute intervals. The AM and PM peak period pedestrian counts are shown in Table 2.
Table 2: Peak Period Pedestrian Counts
Intersection Leg AM Peak Period PM Peak Period
North 10 13
South 9 11
East 13 16
West 11 12
The analyst also projected pedestrian volumes to 2020 to evaluate future pedestrian needs for the build alternatives.
Visualize Crash Data and Identify Safety Issues
The analyst reviewed existing crash data and performed a site visit for the intersection of Smith Road and Main Street to
determine crash patterns. Based on the review of the most recent three years of data, illustrated in Figure 8 and Figure
9, the analyst documented the following patterns and observations:
• 22 crashes (17 PDO; 5 FI) occurred at the intersection over the most recent three years (2015-2017), equating to 7.3 crashes
per year (5.7 - PDO; 1.7 FI).
• Rear-end crashes are the most prominent crashes (37% of crashes), followed by sideswipe crashes (27% of crashes). Rear-
end crashes are typically the most common crashes at signalized intersections.
• Observations at the intersection indicated that no retroreective backplates exist on the signal heads. There are signal
heads for every lane in both the northbound and the southbound approaches. There are also signal heads present for the
eastbound and westbound approaches. All trac signal lenses are LED and 12 inches in diameter.
• The eastbound and westbound left-turn movements are currently permissive only. Right turns on red were allowed.
• No pedestrian facilities are provided at the intersection, and no sidewalks are located along the north and south legs
of the intersection. However, pedestrians were observed crossing all four legs of the study intersection (see Table 2) and
several pedestrian generators are located nearby.
• No pedestrian related crashes occurred at the intersection over the three-year period.
• Overhead lighting is not present at the intersection, but there is existing commercial and corridor lighting surrounding
the intersection.
17
Figure 8: Crashes by Type (2015-2017)
Figure 9: Collision Diagram (2015-2017)
Credit: Toxcel
Image provided by Google 2019
18
The crash data correlations and safety observations summarized here will provide a basis for the proposed development
to consider including safety treatments and strategies, as part of the overall improvements, that can target these issues
and enhance the safety performance of the intersection.
Existing Operational/Queueing Analyses
The analyst evaluated operating and queuing conditions for the impacted intersection of Main Street at Smith Road for
the existing conditions based on Highway Capacity Manual 6th Edition. Table 3 shows results of the capacity analysis
for the existing intersection.
Table 3: Intersection Capacity Analysis (Existing Conditions)
Year 2018 Delay (LOS) Existing Conditions
Peak Hour EB WB NB SB Overall
AM Peak Hour – Average Control Delay in Seconds/
Vehicle (LOS)
48.9 (D) 52.9 (D) 5.0 (A) 4.9 (A) 7.4 (A)
PM Peak Hour – Average Control Delay in Seconds/
Vehicle (LOS)
35.6 (D) 54.9 (D) 13.6 (B) 13.4 (B) 16.7 (B)
Based on the capacity analysis, the intersection overall operates acceptably at LOS B or better. However, the EB and WB
approaches operate at LOS D during both peak hours. Based on the 95
th
percentile queuing analysis, all vehicles are
expected to be accommodated within the existing turn-bay storage lengths.
Future No-Build Analysis
The analyst used the projected volumes to evaluate operating and queuing conditions for the future no-build scenario.
The future no-build scenario, also known as the future background, includes changes in volume due to growth, but
does not changes in volume resulting from a new development. Table 4 shows the results of the capacity analysis for
the intersection.
Table 4: Intersection Capacity Analysis (Future Background)
Year 2020 Delay (LOS) No-Build Conditions
Peak Hour EB WB NB SB Overall
AM Peak Hour – Average Control Delay in Seconds/
Vehicle (LOS)
47.3 (D) 52.4 (D) 5.7 (A) 5.5 (A) 8.0 (C)
PM Peak Hour – Average Control Delay in Seconds/
Vehicle (LOS)
35.5 (D) 60.9 (E) 14.1 (B) 14.0 (B) 17.6 (B)
Based on the capacity analysis, the overall intersection operates at LOS C or better during both peak periods. The
amount of delay experienced by the eastbound and westbound approaches increased, but both approaches still
operate at LOS E in the westbound direction during the PM peak.
19
Analysis
Develop Preliminary Alternatives
Based on the results of the existing conditions and projected growth, the following alternatives are proposed:
• Alternative 1:
o Installation of retroreective backplates on signal heads.
o Installation of pedestrian facilities (e.g., curb ramps and tactile domes, striped crosswalk, and pedestrian
signals) across the north and south legs of the intersection.
o Optimized signal timings.
• Alternative 2:
o Installation of retroreective backplates on signal heads.
o Installation of pedestrian facilities (e.g., curb ramps and tactile domes, striped crosswalk, and pedestrian
signals) across the north and south legs of the intersection.
o Optimized signal timings.
o Installing a southbound-right turn lane.
o Lane geometry conversion of eastbound and westbound approaches from shared left/through lane
+ right lane to left lane + shared through/right lane, which allows phasing change from permitted to
protected-permitted.
Based on the review of the improvements, the installation of the retroreective backplates on signal heads should serve
to improve the visibility of signal heads, and is proven to help reduce crashes, specically rear-end crashes. Despite
the lack of facilities, several pedestrians were recorded crossing at the intersection during both peak periods. The
installation of pedestrian facilities will help connect non-motorized users between the developments and will serve as
a control crossing point for non-motorized users. Additionally, optimizing the signals has proven crash reduction results
while improving overall operations of the intersection. Alternative 2 not only incorporates the changes proposed for
Alternative 1, but also includes additional geometric improvements. The installation of the dedicated right-turn lane
will serve to accommodate the new increase in southbound right-turning trac because of the proposed pharmacy.
The conversion of the lane geometry and phasing on the eastbound and westbound approaches will help to provide
greater and safer throughput for left-turning movements (i.e., highest movements in future conditions), and will help
to accommodate safer conditions for the proposed pedestrian crossings.
Credit: Fotosearch
20
Operational Analysis
The analyst evaluated operating and queuing conditions for the impacted intersection of Main Street at Smith Road
for existing, future background and future total conditions were developed based on Highway Capacity Manual 6th
Edition for Alternative 1 and Alternative 2. Table 5 describes the results of the capacity analysis for Alternative 1 and
Alternative 2.
Table 5: Alternative 1 and 2 Intersection Capacity Analysis (2020 Total Conditions)
Peak Hour EB WB NB SB Overall EB WB NB SB Overall
AM Peak Hour – Av-
erage Control Delay
in Seconds/Vehicle
(LOS)
45.1 (D) 48.6 (D) 8.2 (A) 8.5 (A) 10.9 (B) 62.2 (E) 56.5 (E) 4.0 (A) 4.0 (A) 7.8 (A)
PM Peak Hour – Av-
erage Control Delay
in Seconds/Vehicle
(LOS)
56.0 (E) 50.2 (D) 18.1 (B) 19.5 (B) 23.0 (C) 58.6 (E) 57.0 (E) 5.5 (A) 5.6 (A) 12.0 (B)
Based on the results of the capacity analyses, Alternative 2 performs slightly better than Alternative 1, likely due to the
additional capacity in the southbound approach and the reconguration of the minor street approaches.
Safety Analysis and Comparison
Following the completion of the operational analyses, the analyst evaluated the safety benets of both proposed
alternatives using IHSDM and the CMF Clearinghouse to calculate the associated safety benets. CMFs were applied
separately in order to estimate the safety benets associated with the installation of retroreective backplates and the
installation of pedestrian facilities, which cannot be quantied in IHSDM.
Predictive Method
The analyst used IHSDM to perform the HSM Part C predictive method and determine the expected crash frequency
of the intersection for Future – No Site Trac – No Build, Future – Site Trac – No Build, Alternative 1, and Alternative 2
(Table 6).
Table 6: 20-Year Crash Frequency Results (IHSDM)
Alternative 2020 through 2040 Total Crash Frequency
Future – No Site Trac – No Build 112.3 (84.58 – PDO; 27.72 – FI)
Future – Site Trac – No Build* 129.91 (98.21 – PDO; 31.70 – FI)
Future – Site Trac – Alternative 1* 129.91 (98.21 – PDO; 31.70 – FI)
Future – Site Trac – Alternative 2 119.66 (91.31 – PDO; 28.35 – FI)
*Future – Site Trafc - No-Build and Future – Site Trafc – Alternative 1 are expected to have the same total crash frequency because neither option
includes geometric changes. The crash frequency for Alternative 1 will change after the application of CMFs, which is shown below.
Alternative 2Alternative 1
Year 2020 Delay
(LOS)
21
CMF Application
Improvements that could not be evaluated within IHSDM were analyzed by applying CMFs separately to the results
obtained from IHSDM. CMFs from the CMF Clearinghouse were selected and applied to crash frequencies provided by
IHSDM. Table 7 illustrates the specic improvements selected and their respective CMF values. In some cases, CMFs
may exist for an improvement, but those CMFs may not be applied in the analysis. For example, the “install high-visibility
crosswalk” is an appropriate CMF for the “installation of a crosswalk” improvement. However, some states may require
a certain CMF quality rating for the analysis. In this example, a three-star rating was the lowest acceptable and the
“installation for crosswalk” improvement has a quality rating of two stars. Therefore, the CMF for “install high-visibility
crosswalk” was not used. As a second example, a CMF for “optimizing signals” does not exist in the CMF Clearinghouse
and thus this improvement cannot be accounted for in the analysis.
Table 7: Proposed Intersection Modications and CMFs
Countermeasure CMF Description
CMF ID &
CMF Value
Clearinghouse
Quality Level
Crash Applicability
Description
Included In the
Analysis?
Installation of crosswalk
“Install High-Visibility
Crosswalk”
CMF ID: 4123
0.60
2 stars
Vehicle/Pedestrian
Crashes
No
Install pedestrian
countdown timer
“Install Pedestrian
Countdown Timer”
CMF ID: 8790
0.912
3 stars
All crashes at the inter-
section
Yes
Add retroreective
backplates
“3-inch yellow retro-
reective sheeting to
signal backplates”
CMF ID: 1410
0.85
4 stars
All crashes at the inter-
section
Yes
Optimize Signals CMF does not exist N/A N/A N/A No
Applicable Crash Calculations
Based on the crash applicability description (Table 7), crash reduction factors of 0.912 (installing pedestrian countdown
timer) and 0.85 (installation of retroreective backplates) were applied to all crashes. Because both CMFs can be applied
to all crashes, a composite CMF was calculated as outlined in the HSM. Table 8 represents the combined CMF for both
installing pedestrian signals and installing retroreective backplates.
Table 8: Composite CMF and Applicable Crashes
Countermeasure Crash Modication Factor Applicable Crash Percentage
Install pedestrian countdown timer + install
retroreective backplates
0.78
1
100% of crashes
1
Composite CMF was calculated by multiplying 0.912 and 0.85 crash modication factors for installing pedestrian countdown timer and adding
retroreective backplates, respectively.
22
Safety Performance Evaluation
The CMFs are then applied to the results of the predictive method to determine the nalized crash reduction over the
anticipated 20-year service life. Table 9 indicates the crash reductions resulting from the application of the two CMFs.
The equations below illustrates the calculation for determining the total crash frequencies and total crash reduction
percentages for each alternative.
Table 9: Crash Reduction Results with Predictive Method and CMF Application
Alternative
20-Year
IHSDM Crash
Frequency
(From Table
7)
Composite
Crash Modi-
cation Factor
Applicable
Crashes (%)
Total Crash
Frequency
with CMF (20
Years)
Total Crash
Reduction
(Count, 20
Years)
Total Crash
Reduction
(Percentage,
20 Years)
Future – Site Trac -
No-Build
129.91 (98.21 –
PDO; 31.70 – FI)
N/A N/A N/A N/A 0%
Future – Site Trac -
Alternative 1
129.91 (98.21 –
PDO; 31.70 – FI)
0.78 100%
101.3 (76.6 –
PDO; 24.7 – FI)
28.6 (21.6 – PDO;
7.0 – FI)
22.0%
Future – Site Trac -
Alternative 2
119.66 (91.31 –
PDO; 28.35 – FI)
0.78 100%
93.3 (71.2 –
PDO; 22.1 – FI)
36.6 (27.0 – PDO;
9.6 – FI)
28.2%
Based on the safety analysis, 22.0% and 28.2% total crash reduction percentages are expected for Alternatives 1 and
2, respectively.
)
Alternative 1 Total Crash Frequency = 129.91
*
0.78
*
100%
Alternative Crash Frequency =
(Alternative IHSDM Crash Frequency)
*
(CMF)
*
(Percentage of Applicable Crashes)
Alternative 1 Total Crash Reduction Percentage =
*
100%
101.3 - 129.9
129.9
)(
Total Crash Reduction Percentage =
Alternative Crash Frequency - No Build Crash Frequency
No Build Crash Frequency
(
23
Benet-Cost Analysis
The analyst conducted a benet-cost analysis to quantify the impacts of each alternative. A severity-weighted crash
cost was developed for fatal and injury crashes using three years of crash data and the costs shown in Table 10.
6
The
severity-weighted crash cost was calculated to be $446,212 (2016 dollars). The crash cost used for property damage
only crashes was $11,900 (2016 dollars).
Table 10: Comprehensive Crash-Level Costs
Severity
Comprehensive Crash-Level Cost
(2016 dollars)
K $11,295,400
A $655,000
B $198,500
C $125,600
O $11,9 0 0
The present value of the cost savings due to the reduction in crashes was found for each alternative using a seven
percent discount rate, and benet-cost ratios were calculated (Table 11).
Table 11: Benet-Cost Analysis Comparing Alternative 1 and Alternative 2 to the Future No-Build alternative
Alternative
Total Crash Reduction
Dierence
Present Value of Crash Cost
Savings (20 Years)
Total Estimated
Costs
Benet-Cost
Ratio
Alternative 1 28.6 (21.6 – PDO; 7.0 – FI) $2,015,410 $115,000 17. 5
Alternative 2 36.6 (27.0 – PDO; 9.6 – FI) $2,745,382 $150,000 18.3
Preferred Alternative
The present value of the crash cost savings was found to be $2,015,410 and $2,745,382 over the course of the 20-year
period resulting in benet-cost ratios of 17.5 and 18.3 for Alternatives 1 and 2, respectively. Based on the results of the
benet-cost analysis and the operational analysis, the analyst recommended implementing Alternative 2 given the
greater benet-cost ratio and improved operations.
6 Harmon, T., Bahar, G., & Gross, F. (2018). Crash Costs for Highway Safety Analysis (Final Report No. FHWA-SA-17-071). Washington, DC. Retrieved
from https://safety.fhwa.dot.gov/hsip/docs/fhwasa17071.pdf
24
Key Findings
This analysis addressed the trac-related impacts associated with the proposed 12,000 square foot pharmacy on the
northwest corner of the Main Street and Smith Road. The following conclusions are based on the conducted typical
capacity and safety analyses:
• The project is expected to generate 23 net new AM peak hour trips and 63 net new PM peak hour trips. The corresponding
peak pedestrian counts at the intersection are 43 in the AM peak hour and 52 in the PM peak hour.
• The existing intersection has 22 crashes over the most recent three years (2015-2017), with rear-end crashes (37%) being
the most common type. Patterns of crashes showed a high presence of rear-end crashes on the northbound and
southbound approaches.
• Intersection capacity analyses indicate that the study intersection of Main Street and Smith Road is expected to operate
at a LOS C or better during the PM peak hour for the intersection overall in each scenario.
• Results of the 95th percentile queuing analysis indicate, under future build total conditions, that existing lane
congurations are expected to be accommodated within provided storage lengths.
• Based on the results of the benet-cost analysis, Alternate 2 was recommended as the preferred alternative. Alternative
2 includes adding a southbound right turn lane, reconguring the lanes on the eastbound and westbound approaches,
the addition of pedestrian facilities at the intersection, signal optimization, and enhancing signal conspicuity.
• An overall crash reduction of 28.2% is expected for the preferred alternative. A total savings of approximately $2,745,382
is expected over the course of the 20-year period resulting in a benet-cost ratio of 18.3 for the preferred alternative.
Conclusion
New developments often can be controversial. Communities believe there will be an increase in trac and disruptions
to the existing road network as a result of the development. However, including safety analysis and providing
recommendations that will benet both non-motorized and motorized users could help inuence the local community
view of the project and could make the approval process more manageable for the developer. Additionally, without
a safety analysis, a TIA does not properly identify the true impacts of a proposed development. A safety analysis can
provide value to the public by identifying potential safety issues imposed on roadways and intersections from the new
development. Basic DDSA techniques can be integrated into the development of TIAs by analysts without expending
a signicant level of eort.
FHWA-SA-19-026
