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FHWA Highway Safety Programs

Appendix D – Gaps and Needs in Research

A list of gaps and needs relating to speed management, effectiveness of countermeasures, and speeding-related crash risks emerged during the comprehensive literature review, crash data analysis, and focus group interviews.

The following table lists some critical gaps in our understanding of the problem and initiatives needed to more effectively manage speed to improve roadway departure, intersection, and pedestrian and bicyclist safety.

Gaps/Needs Overall Speed Roadway Departure Focused Intersection Focused Pedestrian/ bicyclist Focused
A better understanding of the relationship between travel speed and crash risk is needed to better define unsafe travel speeds and estimate changes in safety due to changes in speed that are expected from implementing engineering countermeasures. X      
The long-term effects of many traffic engineering and design measures on reducing speed are not well defined or evaluated. X      
NCHRP Report 613, Guidelines for Selection of Speed Reduction Treatments at High Speed Intersections, presents a wide variety of treatments for reducing speed at intersections, but insufficient data is published on the long-term speed and safety impacts. Rigorous evaluation is needed to better understand the safety effects of the following treatments:
  • Dynamic warning signs
  • Longitudinal rumble strips
  • Wider longitudinal pavement markings
  • Approach curvature
  • Splitter islands
  • Speed tables and raised intersections
  • Reduced lane width
  • Visible shoulder treatments
Current speed activated warning systems on curves do not take into account vehicle type or weather conditions   X    
The effects of variable speed limits on operating speeds, compliance, and safety are not well known and are needed to advance the concept, especially in work zones and for adverse weather conditions. X      
Develop strategies for the mitigation of crash risk of trees, utility poles, and other fixed objects in the clear zone   X    
Research is needed to determine the effects of wider edge lines or narrowing lane width X      
The safety effectiveness of posting appropriate speed limits and advisory speeds on intersection approaches has not been quantified for signalized and unsignalized intersections     X  
The effects of variable speed limits at intersections on operating speeds, compliance, and safety are not well known and are needed to advance the concept.     X  
The reduction of speed limits and use of advisory speeds at intersections have little effects on speed, and their impact on safety is not known.     X  
A rigorous evaluation of the safety effects of automated speed enforcement at intersection in the United States is needed to convince legislators and transportation officials of its efficacy. Also, a mechanism is needed to ensure engineering and traffic investigations are performed prior to implementation of automated speed enforcement.     X  
The inferred design speed profile based on American Association of State Highway and Transportation Officials (AASHTO) criteria and average driver, vehicle, and road friction characteristics should be developed and incorporated into the IHSDM and related training. X      
Operating speed estimation models based on roadway design characteristics are available for two-lane highways to assist designers in assessing design consistency based on the magnitude of speed reduction on curves relative to the approach road section. Similar operating speed estimation models need to be developed for rural multilane roadways and incorporated into the Interactive Highway Safety Design Model (IHSDM). In addition to assessing design consistency and potential safety problems on curves, the model would also be helpful in assessing the need for climbing lanes, justification of maximum grades, and evaluating proposed capacity- expansion projects.   X    
National roadway design guidance does not provide criteria for design speeds above 80 mph.
  • Brake reaction time and deceleration rate for drivers and various vehicle types with initial speeds of 80 mph and higher are needed to verify whether current values used in road design are sufficient.
  • Detection and recognition distance between a high speed vehicle and a slower moving vehicle are needed to assess whether they are sufficient to permit evasive maneuvers or stopping.
  • Additional research is needed to determine the potential for rollover and skidding at the higher design speeds on curves, especially as applicable to ramp design.
  • Research is needed to determine side friction demand on horizontal curves on high-speed roadways and how much, if any, drivers slow down.
  • Evaluating the impacts from raising speed limits to 80 mph and higher on roadside safety features.
X X    
Roadside Safety Analysis Program (RSAP), an encroachment-based computer software tool for cost-effectiveness evaluation of roadside safety improvements, includes speed limit as variable affecting encroachment speed, angle and severity, but is largely based on data collected on roadways with speed limits of 70 mph or less. There is a need to develop encroachment speed or angle distributions for roadways with prevailing speeds of 75 mph and higher and identify roadside slopes and ditch configurations that are recoverable, traversable, and non-traversable at high speeds.   X    
The safety impacts of speed limit changes based on USLIMITS2 need to be evaluated; providing before and after studies of corridors when agency used USLIMITS2 as basis for setting speed limit; success stories. X      
A national speed limit database and a curve geometry database are needed to support advanced safety applications such as in-vehicle speed monitoring and curve speed warning. X X    
As connected vehicle (CV), vehicle-to-infrastructure (V2I) and related technologies develop, researchers and developers need to consider integrating solutions for addressing speed and roadway departure, intersection, and pedestrian and bicyclist crashes. X X X X
NCHRP Report 731, Guidelines for Timing Yellow and All-Red Intervals at Signalized Intersections, and a draft ITE proposal recommended practice advises that yellow change intervals be timed using the speed limit + 7 mph to approximate the 85th percentile speed if speed data are not measured and that 1 second be subtracted from the calculated red clearance time to account for driver start-up delay in entering the intersection after green onset. The crash effects of retiming yellow change and red clearance intervals following the new guidelines need to be quantified and crash modification factors updated for inclusion in the HSM.     X  
A much better understanding of perception-reaction, deceleration, red-light running behaviors, etc. at signalized intersections could be determined using the 2nd Strategic Highway Research Program Naturalistic Driving Study (SHRP2 NDS) data.     X  
More robust research on vehicle or speed activated devices is needed. X      
There is a need for a toolbox of countermeasures and details of the effectiveness of each on speed management. X X X X
A better understanding of the relationship between travel speed and crash risk is needed in low and moderate speed environments to better define unsafe travel speeds and estimate changes in pedestrian safety, due to changes in speed that are expected from implementing engineering countermeasures.       X
Speed prediction models are needed for urban streets to assist designers in developing context-sensitive road designs and incorporating features that result in operating speeds that are consistent with the target speed environment. X     X
The engineering toolbox for managing speed is limited, except for residential streets. Cost-effective methods are needed to better manage speeds on urban arterial streets. X      
USLIMITS2 does not address school zones or provide guidance on actions to take if the recommended speed limit is higher than desired. There is a need to incorporate a countermeasure selection tool such as the Pedestrian Safety Guide and Countermeasure Selection System (PEDSAFE) and speed prediction models that could assist users in reducing the road speed to a level that would support a lower speed limit.       X
Speed management needs to be integrated into current training courses on pedestrian safety in order to reach a broader audience and achieve positive outcomes.       X
Research recently recommended bicycle lane widths and parking lane widths for low- speed streets. There is a need to develop guidance for bicycle lane widths for various roadway characteristics based on vehicle speeds and grade.       X
There needs to be more guidance on determining the best pedestrian safety tool or countermeasure for a specific condition       X
There is a need for research to determine what countermeasures can be used to reduce pedestrian crashes cause by turning vehicles at intersections.       X
Research is needed to determine if there is a correlation between the effect of signal timing and whether pedestrians choose to wait and cross at the intersection or instead, cross at midblock locations or illegally.       X