Highway Safety Manual Case Study 3: 

Using Predictive Methods for Alternative Selection 

in Florida 

HSM Reference 

The American Association of State Highway and Transportation Officials’ (AASHTO’s) 

Highway Safety Manual (HSM) Part C Predictive Method (Chapters 10-12) estimates crash 

frequency and severity. The predictive method uses equations known as Safety Performance 

Functions (SPFs) to estimate the predicted average crash frequency as a function of traffic 

volume and roadway characteristics (e.g., number of lanes, median width, intersection control, 

etc.). The HSM provides SPFs for rural two-lane, two-way roads; rural multilane highways; and 

urban and suburban arterials. The predictive method enables informed decision making 

throughout the project development process, including the selection of alternative roadway 

designs. 

Description 

The Florida Department of Transportation (FDOT) Plans Preparation Manual (PPM) establishes 

design criteria and guidance for FDOT new construction, major reconstruction, and resurfacing 

projects on state maintained roadways. The FDOT’s PPM criteria are generally based on the 

more conservative side of ranges in AASHTO’s A Policy on Geometric Design of Highways and 

Streets – also known as the Green Book – to accommodate Florida’s conditions and place an 

emphasis on safety.1 With the recent publication of the HSM, FDOT initiated a pilot project to 

investigate the differences in project cost based on FDOT PPM standards compared to Green 

Book standards to use as a tool for decision making and alternative selection during the project 

development process, with specific emphasis on the evaluation of safety costs utilizing the HSM 

predictive method. 

Discussion

FDOT District 7 (Tampa area) decided to analyze a corridor widening project on State Road (SR) 

574 using the HSM predictive method. At the western end of the study area, the corridor 

currently tapers from a six-lane section to a two-lane section, and the project involves widening 

SR 574 through the study area from a two-lane road segment to a four-lane divided section with 

center median to relieve congestion. Additionally, the section will be designed to include fourfoot designated bike lanes and a sidewalk on the north side of the highway. The critical pinch 

point of widening this section is a busy signalized intersection located near the project’s 

western terminus. At this intersection, the right-of-way (ROW) necessary to accommodate the 

planned cross-section is constrained by railroad ROW paralleling the alignment on the south 

                                                           

1 Designs not meeting PPM standards are considered design variations, whereas, a design not meeting minimum

AASHTO standards and PPM standards requires a design exception.
    HSM Part C Case Study: Alternative Selection 

2 

and the presence of a post office on the northwest corner. No ROW can be obtained from the 

railroad, and affected ROW at the post office, requires acquisition of not only land but several 

parking spaces from the post office parking lot. Early in the design (while preparing the typical 

section package), a study was conducted to evaluate a design variation from PPM standards, 

which narrowed the project at this intersection to minimize the impact on the post office 

property as much as possible. 

The typical PPM design standard for widening this type of roadway is to construct a raised or 

restrictive median. To accommodate a left turn lane on the eastbound approach of SR 574 at the 

intersection, the required median width for this alternative is 19.5 feet wide. In total, this 

alternative would require a right-of-way acquisition of seven feet wide and 2,000 feet long and 

result in a loss of parking spaces from the post office parking lot. The right-of-way acquisition 

for this design was estimated to be $2.2 million. Although a variation from standard policy, in 

situations where the project is so constrained by right-of-way, it is permitted to consider an 

alternative five-lane section with a continuous two-way-left turn lane (16.5 feet wide). While 

this alternative five-lane section (including the four-foot bike lanes and sidewalk) would still 

require the acquisition of right-of-way from the post-office of over four feet wide, it was 

determined that it would retain the use of the parking spaces, which was the driving cost factor 

in the ROW cost estimation. The right-of-way cost for this section was estimated to be $600,000, 

which is $1.6 million less than the restrictive median section. 

While research and practice have shown the restricted median section is generally the safer 

alternative, per past practices, FDOT would have used the $1.6 million difference in right-ofway costs to help justify the selection of the five-lane section. However, with the recent 

publication of the HSM, FDOT was able to use the predictive method to quantify the difference 

in predicted crash frequency and compare the two design alternatives. 

FDOT used the predictive method for urban and suburban arterials (Chapter 12) to evaluate the 

predicted safety performance of each alternative over a 20-year horizon. They used the urban 

arterial SPFs (refer to HSM equation and tables) and adjusted for the proposed geometric 

conditions based on the crash modification factors (CMFs) for median width provided in the 

HSM (Table 12-22). Florida is currently in the process of developing local calibration factors 

(using the procedure outlined in HSM Part C, Appendix A); therefore, a default calibration 

factor (Cr) of 1.0 was used in Equation 12-2 for this analysis. 

FDOT developed a spreadsheet to perform the HSM analysis. The predicted average crash 

frequency for urban and suburban roadway segments is calculated by summing the predicted 

average crash frequencies for multiple-vehicle non-driveway collisions, single-vehicle crashes, 

and multiple-vehicle driveway-related collisions (see Figure 1). First, SPFs were applied to the 

forecasted traffic volumes to calculate the predicted number of total, fatal and injury (FI), and 

property damage only (PDO) multiple-vehicle non-driveway collisions for each year (Equation 

12-10 and Table 12-3). The predicted crash frequency for the FI and PDO crashes were then 

adjusted based on a proportion to ensure the sum matched the total number of crashes 

predicted (Equations 12-11 and 12-12). The same process was used for calculating the single 

vehicle crashes (Equations 12-13, 12-14, 12-15; Table 12-5) and the multiple-vehicle drivewayrelated crashes (Equation 12-16; Table 12-7). 
    HSM Part C Case Study: Alternative Selection 

3 

Predicted Total Average Crash Frequency for Road 

Segment Base Conditions (excludes vehicle crashes 

with bike/ped) (Equation 12-4) 

Where, 

Nbrmv = predicted average crash frequency of multiple- 

 vehicle non-driveway crashes 

Nbrsv = predicted average crash frequency of single- 

 vehicle crashes 

Nbrdwy = predicted average crash frequency of multiple- 

 vehicle driveway-related crashes 

Multiple-Vehicle Non-driveway Crashes (Equation 12-10) 

Where, 

AADT = annual average daily traffic (veh/day) 

L = length of roadway segment (miles) 

a, b = regression coefficients (Table 12-3) 

Single-Vehicle Crashes (Equation 12-13) 

Where, 

a, b = regression coefficients (Table 12-5) 

Multiple-Vehicle Driveway Related Collisions (Eqn. 

12-16) 

Where, 

Nj = Number of driveway-related collisions per 

 driveway per year for driveway type j (Table 12-7) 

nj = number of driveways within segment of driveway 

 type j (both sides of road) 

t = coefficient for traffic volume adjustment (Table 12-7) 

Nୱ୮୤ ୰ୱ ൌ Nୠ୰୫୴ ൅ Nୠ୰ୱ୴ ൅ Nୠ୰ୢ୵୷

Nୠ୰୫୴ ൌ exp൫a ൅ b ൈ lnሺAADTሻ ൅ lnሺLሻ൯

Nୠ୰ୱ୴ ൌ exp൫a ൅ b ൈ lnሺAADTሻ ൅ lnሺLሻ൯

Nୠ୰ୢ୵୷ ൌ ෍n୨ ൈ N୨ ൈ ൬ AADT

15,000൰

୲

Once the predicted average crash 

frequencies were calculated, the next step 

was to determine the cost of crashes for the 

20-year period. For this part of the 

analysis, FDOT used the crash costs 

provided in the HSM (Table 7-1), which 

are equal to $158,200 for FI crashes and 

$7,400 for PDO crashes. Crash costs were 

estimated for each year by multiplying the 

crash costs by the predicted average crash 

frequencies for each crash type. The 

annual crash costs were then converted to 

a present value using a four percent 

discount rate. The resulting crash costs for 

the two design alternatives over the 20-

year horizon are provided in Table 1. 

Additionally, Table 1 provides the 

predicted FI crashes and PDO crashes for 

the 20-year period. 

Based on these results, the four-lane 

divided alternative was predicted to have a 

crash cost savings of approximately $4.2 

million compared to the five-lane with 

two-way left-turn lane alternative. A 

benefit-cost ratio was calculated by 

dividing the crash cost savings by the 

difference in ROW costs.2 The resulting 

benefit-cost ratio was equal to 2.64, 

illustrating that the benefit obtained 

through improvement in crash costs more 

than offset the differential in ROW costs. 

The results of this analysis were used to 

justify the additional ROW costs of the 

four-lane divided section. 

                                                           

2 Although there are differences in costs associated with construction, ROW was found to have the most significant

impact, and therefore, ROW was the only cost considered in the economic analysis.

Figure 1: HSM Equations
    HSM Part C Case Study: Alternative Selection 

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Table 1: Crash Costs and Predicted Crash Frequency for SR 574 Design Alternatives – 20-Year Horizon 

Training Needs 

One of FDOT’s staff members who is actively engaged in AASHTO’s Highway Committee on 

Design, conducted training for District Engineers on the use of the predictive method. This 

training enabled the District 7 engineers to apply the predictive method to the SR 574 study. 

Benefits 

The HSM predictive method enables the design engineer to estimate quantitative safety impacts 

of various design alternatives and provide justification for their design decisions. For the SR 

574 study in particular, the loss of parking at the post office necessary to construct the four-lane 

divided section, may have been difficult to justify to the post office and the public based on 

engineering judgment alone. However, the use of the predictive method provided the design 

engineer with quantifiable evidence on why the four-lane alternative is preferred based on the 

crash cost savings. 

Contact 

CRASH TYPE 

4 LANE, DIVIDED 5 LANE WITH TWLTL 

Crash 

Costs

Predicted Crash 

Frequency Crash 

Costs

Predicted Crash 

Frequency 

FI PDO FI PDO 

Multi-Vehicle 

Non-Driveway 

$1,492,000 12.8 34.3 $2,856,000 24.2 67.2 

Single Vehicle $155,000 1.2 5.4 $235,000 1.4 15.7 

Multi-Vehicle 

DrivewayRelated 

$561,000 4.6 11.6 $3,337,000 27.3 74.1 

Total $2,208,000 19.6 51.3 $6,428,000 52.9 157.0 

David O’Hagan 

Florida Department of Transportation 

(850)414-4283 

david.ohagan@dot.state.fl.us