A Safe System-Based Framework 

and Analytical Methodology for 

Assessing Intersections

FHWA Publication No.: FHWA-SA-21-013

FHWA Contact: Jeffrey Shaw, HSST, (202) 738-7793, jeffrey.shaw@dot.gov

This document is a technical summary of the Federal 

Highway Administration report “A Safe System-Based 

Framework and Analytical Methodology for Assessing 

Intersections” (FHWA-SA-21-008). 

OBJECTIVE

In the United States, the Safe System approach represents a 

paradigm shift in how road safety is addressed. Foundational to 

the Safe System approach is that no person should be killed or 

seriously injured when using the road system, and that it is a 

shared responsibility by all parties involved to ensure this 

becomes reality. From a roadway infrastructure perspective, a 

Safe System approach involves managing the circumstances of 

crashes such that the kinetic energy imposed on the human body 

be kept at levels that are tolerable in terms of survivability and 

degree of harm. At an intersection, this challenge is 

characterized through managing speed and crash angles, as well 

as considering risk exposure and complexity. This project

developed a Safe System for Intersections method that can be 

applied at a project level and be incorporated into an Intersection 

Control Evaluation alternatives screening process to provide 

another metric for safety.

INTRODUCTION

Countries with Vision Zero initiatives have identified key 

principles to guide their national approaches to road safety 

management— Safe System approaches that result in a Safe 

System. While Vision Zero describes the goal and Safe System

describes the approach, both accept the premise that crashes will 

not be completely avoided, therefore managing the mechanical 

forces in those crashes becomes the priority. Johansson (2009) 

further elaborated this point, explaining that a Safe System 

approach is one where the basic design and operational 

parameter is to not exceed the “level of violence the human body 

can tolerate without being killed or seriously injured” in the event 

of a crash.

TECHBRIEF

Office of Safety Technologies

1200 New Jersey Avenue, SE

Washington, DC 20590

https://safety.fhwa.dot.gov/intersection/
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In its National Road Safety Strategy (2011-2020), 

the Australian Transport Council (2011) identifies 

three guiding principles to their Safe System 

approach, paraphrased below:

• People make mistakes; such mistakes on the 

road network should not result in death or 

serious injury. 

• There are known physical limits to the amount 

of force the human body can withstand before 

serious injury occurs.

• A Safe System is one where forces in collisions 

do not exceed the limits of human tolerance; 

system planners, designers, and managers 

should therefore consider the physical limits of 

the human body in planning, designing, and 

maintaining roads and vehicles and in managing 

speeds.

Achieving a Safe System depends on contributions 

from the whole transportation system. Safe System 

documentation sometimes represents the whole 

system as interacting principles spanning “safe 

roads,” “safe speeds,” “safe vehicles,” “safe road 

users,” and “post-crash care”. This approach is 

commonly reflected in State Strategic Highway 

Safety Plan (SHSP) efforts, which outline strategies 

for leveraging resources that span engineering, 

education, enforcement, and emergency medical 

services to collectively address safety challenges 

and reduce fatalities and serious injuries (FHWA, 

2017). 

It is not possible to achieve a Safe System through 

road infrastructure planning, design, and operation 

alone. However, road infrastructure characteristics 

such as geometrics and traffic operation and 

control strategies can be assessed from a kinetic 

energy management perspective that is central to 

the Safe System approach. As planned points of 

conflict—including conflicts between vehicles and 

nonmotorized users—intersections and 

intersection safety performance have major 

implications on the safety performance of the 

overall transportation system. Intersection projects 

offer unique opportunities to apply a Safe System 

approach to planning, design, and operational 

decisions. 

METHODOLOGY

A primary objective of this research effort was to 

develop a Safe System for Intersections (SSI) 

analytical methodology that intersection planners 

and designers can readily implement and that 

dovetails with the typical project development 

process—one that incorporates Safe System 

principles and relies upon commonly available 

project-level data. The goal is to provide a technical 

basis by which intersection planners and designers 

can apply kinetic energy management to common 

intersection projects in the U.S. However, the 

method’s framework provides flexibility to 

incorporate broader system efforts and 

characteristics (e.g., users, vehicles, speeds) in the 

future if supporting data are available. The 

following represent key considerations and 

characteristics of the SSI method, with additional 

detail provided in the full report.

Data Needs

In order to make it readily usable for practitioners 

during alternatives screening, the SSI method was 

developed with typically available project data in 

mind: posted speed limit, average annual daily 

traffic (AADT) volumes, and the number of through 

lanes on the intersecting roads. There are also 

several optional inputs that, if available, will make 

the analysis more project specific. Some of these 

optional inputs (e.g., vehicle speeds for different 

intersection movements and volumes of 

nonmotorized users) are central to Safe System 
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principles but have not been as utilized or explored 

historically by the research and practitioner 

communities. The SSI method offers assumptions 

and default values for their use, but agencyprescribed or project-specific values could also be 

applied. 

Conflict Point Identification and 

Classification

A conflict point is any location where the paths of 

road users coincide (FHWA, 2019). By their nature 

as planned points of conflict, intersections 

represent concentrated groupings of conflict 

points. The SSI method categorizes conflict points 

as either crossing, merging, diverging, or 

pedestrian conflict points. The SSI method 

currently assumes that bicyclists follow the same 

paths as pedestrians through intersections; future 

enhancements to the method could incorporate 

additional layers of vehicle-bicycle conflict points 

that depend on the selection of bicycle 

accommodation through the intersection. The SSI 

method does not consider rear-end conflicts that 

result from speed differentials that arise from 

traffic congestion or deceleration and stopping due 

to traffic control devices (i.e., yield signs, stop 

signs, and traffic signals) but does consider rearend conflicts resulting from speed differentials at 

diverging conflict points where vehicles making 

different movements have different speeds.

Conflict points can be identified on a movement 

basis or on a lane-by-lane basis. Since this initial SSI 

method is intended for use in alternatives 

screening when exact lane arrangements may not 

be known, the SSI method identifies conflict points 

on a movement basis. While movement-based 

conflict points are not dependent on the number of 

lanes or presence of auxiliary lanes in an 

alternative, they are disaggregated by each 

movement combination. For example, where 

pedestrians cross a minor road leg of a four-legged 

intersection—where left turns, right turns, and 

through movements can be made onto and from 

the minor road—there would be six total conflict 

points. Figure 1 shows an example of the 

movement-based conflict points for a traditional 

minor road stop control intersection. The full 

report illustrates movement-based conflict points 

for various intersection alternatives that State 

agencies with Intersection Control Evaluation (ICE) 

policies commonly consider as part of a Stage I ICE.

Exposure 

The likelihood of a crash at a given conflict point is 

related to the number of conflicting movements 

that pass through that conflict point. The SSI 

method accounts for this concept of exposure 

through an exposure index, which is estimated for 

each conflict point. The SSI method adopts an 

exposure index definition from Hakkert & Mahalel 

(1978) in which the exposure index at a given 

conflict point is simply the product of vehicle or 

nonmotorized user daily volumes passing through 

that conflict point. The individual conflict point 

exposure indices can be summed across all conflict 

points of a certain type at an intersection to 

compute the total exposure for each conflict point 

type (e.g., total exposure through all crossing 

conflict points or total exposure through all 

merging conflict points).
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Conflict Point Severity

The SSI method defines conflict point severity as 

the estimated probability of at least one fatal or 

serious injury (P(FSI)) as a result of a crash between 

conflicting road users making the typical 

movements that define the conflict point. The SSI 

method defines injury severity on the maximum 

abbreviated injury scale (MAIS), which is based on 

information provided by trained medical 

professionals following an assessment of a 

patient’s injuries at the hospital (Burch et al., 

2014). MAIS classifications of injury severity may 

be more consistently coded within a State, across 

States, and over time than injury determinations 

made by police officers at the scenes of crashes. 

The ability to make a direct correlation to a 

person’s probability of survival is another benefit of 

the MAIS scale. The SSI method defines fatal and 

serious injuries as injuries with MAIS scores of 3 or 

above.

The full report details the calculations used to 

estimate conflict point severity for the SSI method. 

Application of the P(FSI) models to determine 

conflict point severity requires estimates of vehicle 

speeds through each conflict point and—for 

vehicle-to-vehicle conflicts—an estimate of the 

collision angle between the vehicles. With limited 

existing research into speed prediction at 

intersections, the SSI method adopts a simplified 

set of speed assumptions to cover the various 

maneuvers at intersections that can be adjusted 

based on local knowledge or any data that become 

available in the future. The collision angle used to 

compute conflict point severity in the SSI method is 

based on the convention established in Jurewicz et 

al. (2017); to facilitate efficient application to a 

variety of intersections, the SSI method identifies 

five categories of potential collision types and 

proposes a typical range of collision angles for each 

based on typical movement arrangements at 

intersections. They do not account for intersection 

Figure 1. Graphic. Diagram of movement-based conflict points for

Traditional Minor Road Stop Control intersections.
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skew or other context-specific geometrics but 

could be adjusted if that information is available.

Movement Complexity, User Workload, 

and the SSI Score

Concepts related to user behavior and the 

workload imposed (or mitigated) by the 

intersection design and operations will also affect 

the crash risk per given level of exposure. As such, 

the SSI method considers features corresponding 

to the overall intersection form and size that could 

impact the task complexity for users making 

specific movements at an intersection. The 

complexity concepts are applicable at the 

movement level, with the corresponding 

complexity factors being applied to the appropriate 

conflict points along that same movement. The SSI 

method derives two main complexity factors. The 

first intersection complexity factor captures 

complexity added by the characteristics of 

conflicting traffic, while accounting for how much 

of that complexity is moderated by the type of 

traffic control. This first complexity factor applies 

to both vehicle and nonmotorized movements 

through an intersection and therefore to the 

vehicle-vehicle and vehicle-nonmotorized conflict 

points along those movements. The second 

intersection complexity factor is an additional 

nonmotorized complexity factor. This second factor 

accounts for indirect and nonintuitive movements 

at an intersection that may present additional 

complexity for pedestrians and cyclists. The SSI 

method assumes intersection attributes associated 

with lower levels of complexity for all users will 

ultimately bring it into closer alignment with a Safe 

System.

Results and Potential Use

The results of applying the SSI method include 

multiple measures of effectiveness (MOEs) and a 

proposed set of SSI scores. The MOEs include the 

exposure through different conflict point types, the 

average P(FSI) for different conflict point types, and 

the average complexity for movements passing 

through different conflict point types. The SSI 

scores are derived based on the combined 

concepts of conflict points, conflict point severity, 

exposure, and complexity and are a means to 

characterize the extent to which an intersection 

alternative in a given context aligns with the 

principles of a Safe System. The score for an 

intersection control alternative ranges from zero to 

100, with higher values representing higher levels 

of Safe System performance (i.e., lower chances of 

fatalities and serious injuries).

The SSI MOEs and the SSI scores can serve as 

additional safety metrics to inform the process of 

screening alternatives and identifying an optimal 

solution for an intersection. A Stage I ICE safety 

analysis provides a basis to characterize safety 

performance of various alternatives. Performance 

analyses that occur during a Stage I ICE may rely on 

both qualitative and quantitative methods. 

Depending on the project intent, the Stage I safety 

analysis is generally meant to determine one of the 

following: 

• If improving safety is the primary need for a 

project, does the intersection alternative 

address the safety need by enhancing safety 

performance? 

• If improving safety is not the primary need for a 

project, does the intersection alternative 

maintain or enhance safety performance?

The SSI MOEs and SSI scores can complement 

crash-based metrics that come from predictive 

approaches like those in the Highway Safety 

Manual (HSM) and Safety Performance for 

Intersection Control Evaluation (SPICE) by:
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• Focusing on fatalities and serious injuries 

defined on the MAIS scale and the key 

mechanisms that lead to these injuries (e.g., 

speeds, collision angles).

• Providing a metric for the safety of 

nonmotorized users while robust crash-based 

metrics are still in development.

• Communicating tradeoffs between vehiclevehicle conflict SSI scores and vehiclenonmotorized conflict SSI scores across 

different intersection alternatives.

The SSI MOEs and SSI scores can also provide 

metrics that consider safety in the absence of an 

HSM or SPICE analysis. This may be valuable in 

cases where it is not possible to conduct crashbased analyses on one or more alternatives, such 

as for atypical or emerging intersection concepts

that are not-addressed by crash-based methods.

Summary and Future Expansion of 

the SSI Methodology

While U.S. intersection planning and design 

practices have incorporated Safe System principles 

to some extent over the last several decades, work 

remains to be done. The SSI method developed 

through this research effort provides an approach 

to characterize intersection alternatives with 

respect to the Safe System principles of simplified 

decision-making and management of impact angles 

and speeds, with the ultimate 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 motorized and nonmotorized 

users. The SSI method is sensitive to volumes, 

vehicle speeds, potential collision angles, and 

geometry. The results of applying the SSI method 

include multiple MOEs 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 I ICE.

Looking to the future, there are multiple 

opportunities to expand the SSI framework. The 

full report details these opportunities, organizing 

them into two categories: 1) SSI enhancements for 

common intersection planning and design 

applications and 2) SSI enhancements for broader 

Safe System implementation.

SSI enhancements for common intersection 

planning and design applications include the 

following: 

• Incorporate more refined identification and 

analysis of pedestrian and bicyclist conflict 

points, with a promising potential approach 

being to develop multiple alternatives for a 

single intersection type that differ by 

pedestrian and bicycle accommodation – e.g., 

Restricted Crossing U-turn (RCUT) with sidewalk 

and on-street bike lanes, RCUT with shared use 

paths, RCUT with sidewalks and separated bike 

lanes and a protected intersection. 

• Expand to other conflict types, such as rear-end 

conflicts that result from speed differentials 

that arise from traffic congestion or 

deceleration and merge and diverge conflicts 

that may vary in their location along an 

intersection approach due to lane changing, 

including weaving movements.

• Develop data and models to support 

intersection speed prediction that would 

provide insights to expected operating speeds 

by intersection type, type of traffic control, 

movement, and traffic volume variations 

throughout the day.
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• Develop, validate and calibrate relationships 

between the SSI metrics/scores and crashbased models (such as those in the HSM) in 

order to better target reductions in fatal and 

serious injury crashes at intersections. 

In addition to use by intersection planners and 

designers within the typical project development 

process, the fundamental building blocks of the SSI 

method would also allow it to incorporate impacts 

of broader system-level policies and characteristics 

on SSI MOEs and SSI scores. Such capabilities could 

help advance stakeholder knowledge of the Safe 

System approach and support continued dialogue 

on steps to achieve a vision of zero fatalities and 

serious injuries in the U.S. The following are ideas 

for broader Safe System implementation:

• The SSI method could be used to explore and 

communicate impacts of effective speed 

management and self-explaining roads policies 

at the intersection level. 

• The SSI method could incorporate different 

aspects of vehicle design, such as vehicle size 

and automated driving system technologies, if 

corresponding data or assumptions are 

available.

• The existing structure of SSI method could 

incorporate user characteristics and behavior in 

an average/aggregate way; at some point, the 

large number of possible user and vehicle type 

combinations could lend themselves to a Safe 

System analysis by a microscopic safety 

simulation, where distributions of user 

characteristics, vehicle characteristics, and user 

arrival distributions are inputs and the 

intersection is modeled in a stochastic way.

References

Australian Transport Council (2011). National Road Safety 

Strategy 2011-2020. Australian Transport Council, 

Canberra, Australia. 

Burch, C., Cook, L., & Dischinger, P. (2013). A Comparison 

of KABCO and AIS Injury Severity Metrics Using CODES 

Linked Data. Traffic Injury Prevention, 15(6), 627-630.

FHWA (2019). Crossover-Based Intersections. 

https://safety.fhwa.dot.gov/intersection/innovative/cross

over/. Accessed September 24, 2020.

FHWA (2017). About the Strategic Highway Safety Plan. 

https://safety.fhwa.dot.gov/shsp/about.cfm. Accessed 

September 24, 2020.

Hakkert, A.S. & Mahalel, D. (1978). Estimating the Number 

of Accidents at Intersection from a Knowledge of Traffic 

Flows on the Approaches. Accident Analysis & Prevention, 

10, 69-79.

Johansson, R. (2009). Vision Zero - Implementing a Policy 

for Traffic Safety. Safety Science, 47, pp. 826-831

Jurewicz, C., Sobhani, A., Chau, P., Woolley, J., & Brodie, C. 

(2017). Understanding and Improving Safe System 

Intersection Performance [Report No. AP-R556-17]. 

Austroads, Sydney, Australia. 

Tingvall, C. & Haworth, N. (1999). Vision Zero-An Ethical 

Approach to Safety and Mobility. Presented at the 6th ITE 

International Conference Road Safety & Traffic 

Enforcement: Beyond 2000, Melbourne, Australia. 

https://www.monash.edu/muarc/ourpublications/papers/visionzero. Accessed September 24, 

2020.
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A Safe System-Based Framework and Analytical Methodology for Assessing Intersections

Researchers—Richard J. (R.J.) Porter, Michael Dunn, Jonathan Soika, Ivy Huang, Dylan Coley, Annette Gross, 

Wes Kumfer, and Stephen Heiny. The Contracting Officer’s Representative and Task Manager was Jeffrey Shaw 

of the FHWA Office of Safety Technologies. This work was performed by VHB under contract 693JJ318F000100. 

Keywords—Safe System, intersection, safety performance, Intersection Control Evaluation, conflict point.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation in the 

interest of information exchange. The U.S. Government assumes no liability for the use of the information 

contained in this document. The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the objective of the 

document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides high-quality 

information to serve Government, industry, and the public in a manner that promotes public understanding. 

Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure 

continuous quality improvement.

JANUARY 2021 FHWA-SA-21-013