A Guide to Applying the Safe System Approach to Transportation Planning, Design, and Operations (2025) / Chapter Skim
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4 Safe System Design
4 Safe System Design
Pages 28-37
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From page 28... ...
, 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.
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From page 29... ...
• 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.
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From page 30... ...
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.
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From page 31... ...
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 90o 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)
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From page 32... ...
The illus trated 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.
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From page 33... ...
◾ 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.
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From page 34... ...
Improving sight Increases awareness distance at by enhancing the intersections by visibility of other − − − Low restricting parking road users at at the corners intersections (daylighting) Installing travel Provides separated lane spaces for people reconfigurations at riding bikes and e- − Low multilane roads scooters, can enable with fewer than people to cross only 20,000 annual one lane of traffic at average daily a time, and prevents traffic rear-end, left-turn, and side-swipe crashes
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From page 35... ...
. How Safety Is Improves IRA, Example Practice Improved Exposure Likelihood Severity PCC, or CD1 Costs2 Installing centerline Provides tactile rumble strips on feedback to undivided motorists who start − − − Medium highways to drift across the centerline, which can prompt drivers to return their vehicle to the travel lane Converting Reduces the speed of conventional vehicles entering the signalized intersection and − High intersections to decreases the angles single-lane at which crashes roundabouts may occur, especially side impact and head-on crashes Installing raised Reduces the severity pedestrian/ of pedestrian and bicyclist crossings bicycle crashes by − − High at driveways, minor encouraging drivers street intersections, to slow down on and midblock approaching non transit stop intersection locations crossings Installing shoulder Provides tactile or edge line rumble feedback to strips with bicycle motorists who start − − − Medium gaps on undivided to drift across the highways edge line and offers bicyclists opportunities to merge into the travel lane without contending with rumble strips Creating "self- Manages speeds via enforcing" road consistent cross designs where local section design − − High roads have narrow within road same lanes and traffic classifications and calming, collector distinct design roads have bicycle between different lanes and safe road classifications pedestrian crossings, and arterial roads severely limit access and provide protected bicycle lanes and pedestrian crossings Note: − = Not applicable.
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From page 36... ...
• Key factors in the future will likely include the potential for favoring more advantaged populations with the safety benefits of roundabouts, the waxing and waning of public interest in roundabout conversions, expansion of roundabout conversions across geographies over time, and more. • Success indicators might include observed reductions in operating speeds within the roundabout, higher rates of drivers yielding to crossing pedestrians versus at midblock crossing locations, decreasing fatalities and serious injuries at roundabouts versus at signalized intersections, community member reports of feeling safer using roundabouts versus signalized intersections, increased pedestrian use of the intersection, and more.
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From page 37... ...
. Step Example Step Elements Improve learning of practice • Learning about practice performance requires consistent measurement performance to make necessary of practice performance indicators such as the proportion of drivers adjustments to shore up practice yielding to crossing pedestrians at the roundabout sites, the number of performance or to change up the serious and fatal injuries at roundabouts versus other intersections, practice altogether.
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