The Safe System Approach represents a paradigm shift in how road safety is addressed in the United States and aligns with the growing number of Vision Zero goals, efforts, and action plans across the country. While Vision Zero describes the goal and Safe System describes the approach, both accept the premise that crashes will not be completely avoided. However, the Safe System Approach posits that no person should be killed or seriously injured if a crash occurs when using the road system, and that it is a shared responsibility of all parties involved to achieve this outcome. For road design, a Safe System Approach involves managing the circumstances of crashes such that the kinetic energy forces imposed on the human body should not result in death or serious injury. At an intersection, this is accomplished by influencing conflict points, speed, and crash angles, and considering exposure and complexity.

Crashes attributed to intersections contribute significantly to traffic fatality and injury numbers in the United States. In 2019, 10,180 people were killed in intersection and intersection-related crashes, which is roughly one-quarter of all roadway fatalities. Additionally, about half of all injury crashes occur at or near intersections. Intersection projects represent a straightforward opportunity to explore how to apply Safe System principles to the project development planning and design decisions in support of Vision Zero, as well as a performance-based approach to safety.

FHWA sponsored the effort to develop a Safe System for Intersections (SSI) framework and methodology, which represents a first step toward the development of objective and implementable analyses that reflect key Safe System concepts. The Safe System assessment of an intersection can serve as an additional metric to inform alternatives analysis and identify an optimal solution for an intersection. In fact, the SSI method can provide a valuable quantitative safety metric in addition to, or in the absence of, the types of crash-based approaches that are the foundation of the Highway Safety Manual.

Bonnie Polin, a State safety engineer from the Massachusetts Department of Transportation (MassDOT), says, “We had an opportunity to test out the SSI approach to evaluate design alternatives for two irregular, nontypical intersections where safety performance function models were not available. The SSI method, along with other approaches such as road safety audits and video analytics using drone technology, can help us gain a more complete safety performance picture to help us identify a preferred alternative that reduces risk and minimizes the potential for harm.”

Explanation of the SSI Method 

The SSI method uses data that are typically available early in a project development lifecycle, including posted speed limit, average annual daily traffic volumes, and the number of through lanes on the intersecting roads. When available, several optional inputs—such as individual movement speeds, daily nonmotorized volumes, turning movement proportions or volumes, and left-turn traffic signal phasing—can be incorporated to enhance the analysis. The SSI method offers assumptions and default values, but agency-prescribed or project-specific values could also be used.

The overall framework and components of the SSI method are based on conflict point identification and classification, exposure, severity, and complexity. With these inputs, it is possible to quantify the degree to which a given intersection alternative is consistent with Safe System principles, and then contrast different alternatives.

The SSI method begins with the identification and classification of conflict points for each intersection alternative. Since exact lane arrangements may not be known at the project development stage, the SSI method identifies conflict points on a movement basis. A conflict point is any location where road users’ paths coincide, categorized as either crossing, merging, diverging, or nonmotorized. This first version of the SSI method assigns both pedestrians and bicyclists to the same path through the intersection, but future enhancements to the method could incorporate additional layers of vehicle-bicycle conflict points depending on the selection of bicycle accommodation through the intersection, such as those described in the FHWA Bikeway Selection Guide (FHWA-SA-18-077). The SSI method also does not currently consider rear-end conflicts that result from traffic congestion or deceleration/stopping because of traffic control devices, but does consider rear-end conflicts that result from speed differentials at diverging conflict points where vehicles making different movements have different speeds.

Once the conflict points are identified and classified, the SSI method characterizes exposure, which is the crash likelihood at a given conflict point given the number of vehicles or nonmotorized users that pass through it. To do this, the SSI method employs an exposure index, adapted from past research. The first step involves calculating the product of crossing movements, using vehicle or nonmotorized user daily volumes, through each conflict point. The second step is to sum the results across all conflict points of each type at an intersection to compute total exposure for each conflict point type.

Next, conflict point severity is the estimated probability of at least one fatal or serious injury, or P(FSI), resulting from a crash between conflicting road users making the movements that define the conflict point. The SSI method defines serious injury as an injury with a Maximum Abbreviated Injury Scale score of 3 or above. The SSI method estimates P(FSI) at crossing, merging, and diverging conflict points using an estimated speed for each conflicting movement and an estimated angle between conflicting movements. For nonmotorized conflict points, the SSI method only requires the vehicle speed at the conflict point to compute P(FSI). The full report for this project offers additional detailed explanation of the basis and steps for calculating P(FSI).

Finally, the SSI method accounts for movement complexity using adjustment factors that relate to the conflicting traffic scenarios. The conflicting traffic complexity factor is based on the relative complexity due to traffic control (such as permissive versus protected signalized movements), and, for all movements, the number of conflicting lanes and the speed of conflicting traffic. The nonmotorized movement complexity factor addresses complexity specific to nonmotorized movements through the intersection by accounting for indirect and nonintuitive movements at an intersection that may present additional complexity for pedestrians and cyclists. Taken together, the movement complexity factors represent a human factors approach that considers the potential workload imposed on road users as they make specific movements through the intersection.

The SSI Score 

The first step in determining the SSI score is to compute the sum of the exposure-severity-complexity products for all individual conflict points of a specific type and to apply the appropriate adjustment factors. The second step is to convert that computed value to a score that has a range of 0 to 100, with 100 representing the best score possible, equating to the lowest probability of a fatality or serious injury in the event of a crash. The method produces an SSI score for each conflict point type (such as crossing, merging, diverging, nonmotorized) as well as for the intersection overall. The SSI method also yields relative scores for exposure, severity, and complexity in order to provide additional context to the SSI scores.

Example Application of SSI Method 

The example intersection is a signalized suburban intersection of a four-lane major road and a two-lane minor road. Design year traffic volumes are estimated at 25,000 and 20,000 vehicles per day, respectively, on the major and minor roads. The posted speed limits are 45 miles (72 kilometers) per hour on the major road and 35 miles (56 kilometers) per hour on the minor. There are sidewalk facilities along all approaches, and the intersection serves a daily volume of 2,400 nonmotorized road users.

First, a Stage 1 Intersection Control Evaluation (ICE) assessment is completed using the Safety Performance for Intersection Control Evaluation (SPICE) and Capacity Analysis for Planning of Junctions (CAP-X) screening tools (both tools and user guides are available at https://safety.fhwa.dot.gov/intersection/ice). ICE is a data-driven, performance-based framework to screen intersection alternatives and identify an optimal solution, and SPICE and CAP-X are screening-level tools used to characterize safety and operational performance. This assessment produces 11 possible alternatives, including ones that the screening tools did not explicitly model. As a means to a quantitative safety metric when crash-based models are not available, this is an area where the SSI method can be immediately helpful. After the SPICE and CAP-X assessments, the SSI method was applied to these 11 different intersection alternatives to produce SSI scores.

The SPICE results for the example application contain the predicted number of crashes for the design year for both total crashes (all types and severities) and fatal and injury crashes unless there is not an appropriate safety performance function available, as in the case of the 2x2 roundabout. The results show that all the intersection types for which there are predictive methods available have fewer total and fewer fatal and injury crashes than the signalized traditional intersection that is the no-build condition. Based on these SPICE results, no intersection alternatives are dropped from consideration.

CAP-X primarily assesses intersection types by using critical lane analysis to compute the volume-to-capacity ratio given vehicle volume inputs and intersection lane arrangements. Based on these results, it can be seen that all but the unsignalized restricted crossing U-turn (RCUT) intersections would be operationally similar to or better than the existing signalized traditional intersection. Otherwise, all these alternatives are operationally feasible.

Based on the SSI scores for individual conflict point types, seven alternatives have improved SSI scores for the nonmotorized conflict points compared to the signalized traditional intersection (which is the existing/no-build condition). Eight alternatives have improved crossing conflict SSI scores compared to the no-build alternative: partial multilane (2x1) roundabout, median U-turn, full multilane (2x2) roundabout, signalized RCUT, bowtie, unsignalized RCUT, full displaced left turn, and partial displaced left turn. These designs reroute one or more movements at the intersection, removing crossing conflict points, reducing vehicle speeds and angles at crossing conflict points, or both. Because the SSI method is sensitive to conflict point speed, for alternatives that do not afford nonmotorized users the benefit of either low speed through geometry (such as a roundabout) or separated movements through traffic control phasing (such as the signalized alternatives), the nonmotorized conflict scores can be as low as zero.

With the SPICE, CAP-X, and SSI results available, the alternatives can be compared and contrasted further to make a recommendation on which preferred alternative(s) should be carried forward into the next phase of project development. Alongside the SPICE and CAP-X results, the SSI scores point to the “hybrid” 2x1 roundabout or the 2x2 roundabout as the most appealing alternatives. That roundabouts would compete so well based on Safe System principles supports the international literature and experience on both the Safe System Approach and roundabouts.

The unsignalized RCUT shows promising SPICE results in terms of predicted total crashes and predicted fatal and injury crashes and SSI method scores that are comparable to the existing traditional signalized, except for a particularly poor nonmotorized conflict score of zero that brings the overall SSI score down as well. This highlights the importance of giving nonmotorized users greater attention and accommodation at certain alternative intersections that have otherwise been proven to offer significant enhanced safety performance. It is likely that an unsignalized RCUT alternative that reduces speed through nonmotorized conflict points and eliminates indirect or nonintuitive movements could achieve a much better SSI method score and preserve or even enhance the safety performance that has been documented through crash-based studies.

Future SSI Applications 

While U.S. intersection planning and design practices have incorporated Safe System principles to some extent over the last several decades, significant opportunities for advancing the Safe System Approach remain. In addition to MassDOT, other State DOTs are also evaluating the SSI method for their intersection projects, including California, Florida, Washington, and Virginia.

Stephen Read, the safety planning program manager for the Virginia DOT, says, “While Virginia is updating [its] Strategic Highway Safety Plan to be more Safe System centric, VDOT’s Traffic Engineering Division is conducting a review of the SSI method to determine whether it could be incorporated into a new Intersection Control Evaluation program that is also under development.”

Where enough data are available, U.S. experiences with intersection alternatives that simplify road user decisionmaking and manage conflict points, impact angles and speeds have shown safety performance benefits. These safety benefits are typically expressed in the form of crash modification factors (CMFs) derived from retrospective statistical analyses of crash data. The CMFs are usually applicable to the intersection as a whole and reflect overall changes or differences in the number of crashes at the intersection alternative of interest compared to another intersection alternative. In other words, intersection CMFs are often developed with and applicable to an aggregation of crashes resulting from different movements through the intersection, involving different intersection users, and resulting in a range of injury outcomes. For example, intersection CMFs for fatal and injury crashes apply to crashes of all types with injury outcomes ranging from fatal to possible injuries.

As a complement to aggregate crash-based findings such as CMFs, the SSI method provides an approach to characterize intersection alternatives with respect to the Safe System principles of managing impact angles and speeds and simplifying decisionmaking, with the goal of reducing traffic fatalities and serious injuries. The method is applied at the conflict point level and incorporates the characteristics of different movements through the intersection for all road users. The SSI method is sensitive to volumes, vehicle speeds, potential collision angles, and geometry.

The results of applying the SSI method include multiple measures of effectiveness and a set of SSI scores that can serve as additional safety metrics to inform the process of screening intersection alternatives, such as during a Stage 1 Intersection Control Evaluation. Continued advancements in crash reporting, injury surveillance (including linkages between crash reports and hospital records), and more widespread availability of vehicle movement and speed data will enable more empirical connections to be made between SSI scores and fatal and serious injury crash data.

Jeffrey Shaw, P.E., is the Intersection Safety Program Manager and member of the Safe System Working Group for FHWA’s Office of Safety. He earned his B.S. in civil engineering from the Illinois Institute of Technology.

R.J. Porter, Ph.D., P.E., is a contracted highway safety engineer with extensive experience in transportation research, engineering, and education. He has conducted and led a range of research work for USDOT/FHWA, the National Cooperative Highway Research Program, and State agencies. He earned his B.S., M.E., and Ph.D. in civil and environmental engineering from Penn State University.

Michael Dunn is a contracted transportation analyst. He specializes in pedestrian and bicyclist safety and the practical application of Safe System principles. He earned his B.S. in civil engineering from the University of Alabama and his M.S. in civil engineering from the University of Texas at Austin.

Jon Soika, P.E., is a contracted senior transportation engineer. He has contributed to a variety of intersection design and safety projects for Federal, State, and local agencies. He earned his B.S. in civil engineering from Penn State University.

Ivy B. Huang, Ph.D., is a contracted water resources designer and has used her data analysis experience to assist in water resources and transportation projects. She earned her B.S. in civil and environmental engineering from the Massachusetts Institute of Technology and her M.S. and Ph.D. from Stanford University in civil and environmental engineering.

For more information, visit https://safety.fhwa.dot.gov/intersection/ssi/fhwasa21008.pdf for the full FHWA report or contact Jeffrey Shaw at 202-738-7793 or Jeffrey.Shaw@dot.gov.

The authors would like to thank Annette Gross for her contributions to this article.