CHAPTER 4

Safe System Design

Road and street design shapes the behavior of road users from the mode they use to travel, the speed at which they do so, the routes they take to their destinations, to the perception of which road users have priority (legal right-of-way and preferred priority). Design is used to separate users in space and in time to prevent crashes and to minimize the kinetic energy (and therefore risk to human life and health) should a crash occur. Design also influences the delay people experience while traveling, the hazards they encounter, and the number of people who can be physically transported along a road or street within a certain time frame. This influence is compounded by the relative permanency of hard infrastructure—which can be maintained for decades—imparting long-lasting effects on entire populations’ transportation-related choices, behaviors, health, and safety.

By aligning the design of the road system with the goal of fostering a Safe System, there is great potential to prompt road users to adjust their behaviors that inadvertently pose safety risks to themselves and others. In this way, Safe System–informed design promises to greatly enhance the safety of all road users.

The conventional road safety process employed in the United States is based primarily on ensuring the efficient movement of motor vehicle traffic and a reactive approach to resulting traffic injury (Michael et al. 2021). For example, agencies regularly provide roadway capacity based on present and forecasted motor vehicle traffic volumes at peak 15-minute periods aiming to minimize driver delay (e.g., via LOS metrics), even at the expense of potential road improvements that would improve safety and comfort for diverse people using any travel mode. The resultant increases to roadway capacity (e.g., additional lanes, parallel routes) tend to result in an increase in motor VMT and an increase in all road users’ exposure to dangerous, higher-speed traffic (Ding and Taylor 2022).

While section 2B.21 in the Manual on Uniform Traffic Control Devices for Streets and Highways states that “On urban and suburban arterials, and on rural arterials that serve as main streets through developed areas of communities, the 85th-percentile speed should not be used” as the sole criteria for setting speed limits (FHWA 2023), many engineers and state legislators continue to place undue emphasis on the 85th percentile speed as the most important consideration for setting speed limits. This reactive approach to setting speed limits encourages practitioners to reinforce current driver behavior rather than proactively modifying street designs and setting context-appropriate, self-enforcing speed limits, based on factors such as surrounding land access and the level of separation required between motor vehicles and vulnerable road users to prevent serious and fatal traffic injury (Kumfer et al. 2023).

A shift toward a Safe System requires a transition to an approach that prioritizes safety and mobility choices for all road users. In a Safe System, road users’ safety provides the foundation of all design decisions, and mobility choices stem from this safe foundation (Naumann

et al. 2020). This shift has begun at the federal level with recognition of FHWA Proven Safety Countermeasures (FHWA 2021a), and the U.S. DOT adoption of the Safe System approach as the core of the National Roadway Safety Strategy (U.S. DOT 2022). This new safety approach acknowledges human mistakes and vulnerability and requires the design of a redundant system that protects everyone, including the many people who use roads and streets outside of motor vehicles. A paradigm shift in design is necessary and possible, and the following sections outline design-oriented methods, strategies, and practices to employ in advancing toward a Safe System.

4.1 Key Design Strategies

The following design strategies for transportation networks and roadways can encourage practices that contribute to a Safe System. Design strategies in a Safe System include the following:

• Institute self-enforcing roads.
• Design around human tolerances to crash forces.
• Physically separate fast-moving motor vehicles from each other and from vulnerable road users.

Institute Self-Enforcing Roads

Human psychology plays a fundamental role in the safety of road designs. Safe System–informed roadway design presents environments that are self-enforcing, meaning that users are likely to interpret context-appropriate courses of action without the need for explicit signage or overt communication (Theeuwes 2021). For example, the speed at which drivers feel comfortable traveling should be equal to or less than the posted speed limit. If the comfortable driving speed is higher than appropriate for the environmental conditions, then the roadway design can be physically altered to enhance drivers’ awareness of speed, encouraging them to slow down (U.S. DOT 2019).

Another application of self-enforcing design is to intuitively indicate the road-user movements that have the right-of-way (Dumbaugh and Gattis 2005). The path of road users who have priority should be continuous, rather than the path of road users required to yield the right-of-way to others. For example, where a driveway or minor street crosses a sidewalk or cycle track and drivers are required to yield to crossing pedestrians and cyclists, the sidewalk or cycle track should continue at the same elevation above the roadway (producing a continuous sidewalk) as it passes through the conflict area, as shown in Figure 6.

Photo courtesy of Mobycon.

Design Around Human Tolerances to Crash Forces

Self-enforcing road designs can meaningfully reduce the likelihood of serious crashes given their alignment with human perceptual systems. Additionally, reliably protecting users from severe injury requires employing what is already known about the effects of kinetic energy in crashes to inform street design. The limits of the human body to withstand crash forces are well-known (Johansson 2009; Tingvall and Haworth 1999). As crash impact speeds increase, the probability of death or serious injury to pedestrians increases nonlinearly, as illustrated in Figure 7. The mass of the colliding vehicles also contributes to the probability of death, as does the vulnerability of people inside and outside of vehicles. Moreover, road users vary in their respective vulnerability, with older adults more likely to be seriously injured or killed in a crash than younger people (Tefft 2013).

A safe transportation system is designed in such a way that infrastructure can safely accommodate the speed and mass of vehicles. Where motor vehicles and nonmotorized road users share space, the speed of vehicles must be reduced to a point that collisions would not be lethal for the road users who will be present in the area.

For example, in areas where pedestrians would be expected to cross midblock at locations with restricted sightlines (e.g., residential streets) or where motor vehicles operate in the same space as people walking or playing, Safe System design requires that drivers operate their vehicles at or below speed thresholds for human tolerance to blunt force trauma, as depicted in the residential street in Figure 8.

Source: FHWA 2019a.

Photo courtesy of Mobycon.

Beyond impact speed, the types and angle of approach of crashes play a key role in shaping crash outcomes. For example, side-impact collisions between two motor vehicles are survivable at a higher impact speed than head-on collisions (Johansson 2009; Jurewicz et al. 2016). Further, crashes occurring at approach angles of less than 90^o tend to be less severe than those occurring at more than 90^o, which helps to explain why converting traditional signalized intersections to roundabouts often significantly improves safety (Savolainen et al. 2023; Gross et al. 2013). Designers can use this understanding in shaping roadway design to minimize conflicts between road users and their exposure to intolerable crash forces. In rural contexts, the 2+1 road in Sweden depicted in Figure 9 provides a through-function with operating speeds of about 50 mph (80 km/h) and a median cable barrier to prevent head-on collisions and encourages driver alertness via alternating the number of lanes every few kilometers (Belin et al. 2022).

Where drivers are invited to travel at high speeds, it is fundamental to consider the composition of traffic at links and intersections, such as the stopping distances and intersection sight distances of trucks, buses, and sport utility vehicles. If these characteristics are not compatible with the function of the street (e.g., if intersection sight distance limits visibility of a motorcycle using the same segment), the function of the street may need to be changed. Designers can coordinate with planners to determine the roadway function; this function should be designed around known human tolerances to crash forces. Thus, if vehicle-to-vehicle crashes could conceivably happen at angles of 90^o or greater, such as at 4-way uncontrolled intersections, speeds should not exceed 30 mph, and if vulnerable road users are exposed to vehicles, speeds should not exceed 20 mph (Johansson 2009; Soames Job, Truong, and Sakashita 2022). Design tools such as access management, corner radii reduction, and intersection conversions to roundabouts can be used to reduce speeds, change approach angles, or manage user interactions at high-speed angle conflict points.

Source: Potts 2003.

Physically Separate Fast-Moving Motor Vehicles from Each Other and from Vulnerable Road Users

Just as Safe System–aligned design prevents crash forces from severely or lethally injuring road users, a key Safe System design strategy is to physically separate fast-moving vehicle traffic from more vulnerable road users. For example, on roadways designed for motor vehicle travel at 25 mph or higher, a separated, protected space should be provided for people cycling (Schultheiss et al. 2019). Across entire road networks, agencies can unbundle bicycle networks from car networks (FHWA 2019b). Physical protection—with design features like curbs, barriers, planters, or bollards—provides for user safety. Where vulnerable road users are physically separated from motor vehicle traffic—by providing grade separation between road users of different masses and speeds as shown in Figure 10—and sufficient sightlines are provided for drivers to slow to survivable speeds prior to a collision, higher speeds can be safely accommodated. This strategy is consistent with a Tier 1 solution within the Safe System Design Hierarchy—i.e., “Remove Severe Conflicts” (Hopwood, Little, and Gaines 2024).

4.2 Study-Identified Design Practices

These overarching design strategies yield more specific safety design practices. The illustrated design practices in Table 1 align with the strategies of Safe System design in that they are intended to render traveling across entire road networks more intuitive (and thus less prone to recognition errors), more forgiving of mistakes, and in keeping with the human body’s capacity to survive the kinetic energy transferred in crashes.

The team extracted 16 Safe System–aligned design practices from a literature review phase of the research and presented them to safety practitioners via an online survey (see Appendix C for the complete list). Survey participants were asked to rate the safety impact and the financial, social, and political feasibility of each practice based on their professional experience and institutional knowledge. Analysis of the responses revealed a wide range of feasibility scores, and a more modest range of impact scores, which can be found in Appendix C.

In keeping with Safe System principles and design strategies, the team determined whether each design practice would reduce road users’ exposure to severe crash types (e.g., run-off-road,

Photo courtesy of Dan Burden, pedbikeimages.org.

head-on, intersection, pedestrian, bicyclist, or motorcyclist crashes) and the likelihood road users would be involved in one or more of these crash types.

• Exposure reflects the number of people with the potential to be involved in serious crash types.
• Likelihood reflects the probability road users will be involved in a crash.
• Severity reflects the chances a crash will result in a fatality or serious injury to the road users involved.
• Improves injury risk assessment, professional and community coordination, or crash diagnoses reflects a practice’s ability to estimate road-user injury risks associated with land use, policy, or engineering interventions; to improve coordination among professionals in different sectors and with the public; or to uncover contributing factors to serious crash events.

Table 6 provides example design practices and their change mechanisms (i.e., the steps or processes responsible for improving road users’ safety).

Implementing Safe System Design Practices

Safe System design practices center around strategies to advance self-enforcing roads, structure street networks around human tolerance to crash forces, and where feasible, physically separate travel modes of different sizes, masses, and directions.

To begin implementing Safe System–aligned design practices, consider following these steps and substeps:

1. Identify at least one significant safety problem. Identification and prioritization of a safety problem might be based on the following:
   • – Severity and magnitude of the safety problem (e.g., in the region, across the network, along a corridor)
   • – Disproportionate harm that the problem imposes on some community members
   • – Stated importance of addressing the problem, according to community representatives
   • – Availability of resources to address the problem
2. Once a safety problem has been identified and prioritized, ask the following two questions:
   • –To what extent does a proposed practice align with Safe System–aligned design strategies?
     • Institute self-enforcing roads
     • Design around human tolerances to crash forces
     • Separate fast-moving motor vehicles from vulnerable road users
   • – To what extent does a proposed practice address the following?
     • Road users’ exposure to serious crash forces
     • The likelihood of serious crashes
     • The severity of crashes when they occur
     • Improvements to injury risk assessment, professional and community coordination, or crash diagnoses

For example, in step 1, if a safety team identifies and prioritizes addressing high-speed angle crashes, they might pursue converting conventional signalized intersections to single-lane roundabouts given the severity and magnitude of this safety problem and the disproportionate harm that this problem imposes on some community members.

Then, in step 2, a safety team might conclude that roundabout conversions align with the Safe System strategy to design around human tolerances to crash forces. They might also conclude that roundabout conversions can significantly reduce the severity of crashes when they occur by reducing impact speeds and crash angles within the roundabout.

Table 6. Safe System design practices.

Example Practice	How Safety Is Improved	Exposure	Likelihood	Severity	Improves IRA, PCC, or CD1	Costs2
Incorporating road safety audits in project development and design phases	Helps identify safety concerns prior to construction	−	−	−		Low
Installing poles that break away when struck	Offers forgiving infrastructure in the event drivers run off the road	−	−		−	Medium
Installing right-in/right-out junctions that only allow vehicles to enter and exit from the right	Reduces road-user conflicts by channelizing vehicle turning movements and eliminating some crossing conflicts			−	−	High
Installing cable barriers on the edges and in the medians of rural roads	Separates users in space to catch and protect drivers who drift over the centerlines or edges of roads				−	High
Installing permanent barrier-protected bike lanes on arterial roads	Separates users in space with a physical separation between faster-moving vehicle traffic and slower-moving bicycle traffic				−	High
Providing pedestrian/bicycle bridges or daylit tunnels at intersections	Physically separates pedestrians and bicyclists from through and turning vehicle movements at intersections				−	High
Setting default local street travel lane widths to 10 ft.	Manages vehicle operating speeds	−			−	Low
Improving sight distance at intersections by restricting parking at the corners (daylighting)	Increases awareness by enhancing the visibility of other road users at intersections	−		−	−	Low
Installing travel lane reconfigurations at multilane roads with fewer than 20,000 annual average daily traffic	Provides separated spaces for people riding bikes and e-scooters, can enable people to cross only one lane of traffic at a time, and prevents rear-end, left-turn, and side-swipe crashes				−	Low

Example Practice	How Safety Is Improved	Exposure	Likelihood	Severity	Improves IRA, PCC, or CD1	Costs2
Installing centerline rumble strips on undivided highways	Provides tactile feedback to motorists who start to drift across the centerline, which can prompt drivers to return their vehicle to the travel lane	−		−	−	Medium
Converting conventional signalized intersections to single-lane roundabouts	Reduces the speed of vehicles entering the intersection and decreases the angles at which crashes may occur, especially side-impact and head-on crashes				−	High
Installing raised pedestrian/bicyclist crossings at driveways, minor street intersections, and midblock transit stop locations	Reduces the severity of pedestrian and bicycle crashes by encouraging drivers to slow down on approaching non-intersection crossings	−			−	High
Installing shoulder or edge line rumble strips with bicycle gaps on undivided highways	Provides tactile feedback to motorists who start to drift across the edge line and offers bicyclists opportunities to merge into the travel lane without contending with rumble strips	−		−	−	Medium
Creating “self-enforcing” road designs where local roads have narrow lanes and traffic calming, collector roads have bicycle lanes and safe pedestrian crossings, and arterial roads severely limit access and provide protected bicycle lanes and pedestrian crossings	Manages speeds via consistent cross-section design within road same classifications and distinct design between different road classifications	−			−	High

Note: − = Not applicable.

¹ IRA = injury risk assessment, PCC = professional and community coordination, CD = crash diagnoses.

² Costs correspond to the total financial cost associated with a policy or practice, including labor, equipment, and infrastructure (Low ≤ $100k; Medium = $100k−$1 million; and High ≥ $1 million in total or per year).

At this point, a team should reflect on whether a selected safety practice: (1) aligns with one or more Safe System design strategies; (2) can significantly reduce the likelihood of users’ exposure to severe crash forces or enhance injury risk assessment, professional and community coordination, or crash diagnoses; and (3) is feasible given available resources to institute the practice. If the team concludes that all three criteria are satisfied, the practice should be considered for implementation, and the safety team could follow the steps outlined in Table 7. However, if one or more of these three criteria are not satisfied, teams are advised to start over from step 1 until all three criteria are satisfied.

Consider the example of a safety team looking to convert select signalized intersections to roundabout designs. Table 7 provides recommended steps to implement this safety practice along with elements to consider within each step.

Table 7. Design practice implementation steps and example elements.

Conclusion

The shift toward Safe System–aligned design is possible and necessary for the United States to realize zero deaths and serious injuries on the nation’s roadways. Road and street design shapes decisions about how people decide to travel, the speed at which they travel, and their opportunity to respond to potential conflicts. By aligning the design of the road system with the goal of fostering a Safe System, there is great potential to prompt road users to adjust routine behaviors that inadvertently contribute to a serious crash. Safe System–informed design can reshape the nation’s streets and transportation system, creating gradual and long-lasting effects on entire populations’ transportation-related choices, behaviors, health, and safety. Operating and maintaining these safety effects is the subject of Chapter 5, which covers Safe System–aligned operations and maintenance strategies and practices.