American Association of State Highway and Transportation Officials

Special Committee on Research and Innovation

FY2023 NCHRP PROBLEM STATEMENT TEMPLATE

Problem Number: 2023-G-14

Problem Title

Evaluate the Benefits of Increasing Clear Zone at Higher Speed/Traffic Volume/Crash Locations

Background Information and Need For Research

The current edition of the AASHTO Roadside Design Guide (RDG) provides guidance to assist transportation agencies develop standards and policies for determining the widths of clear zones along roadways based on design speed, traffic volume, roadside slope, and curvature. By providing clear zones, transportation agencies can increase the likelihood that a roadway departure results in a safe recovery and mitigate the severity of crashes that do occur. The development and use of the clear zone concept has dramatically reduced the number of crashes, injuries, and fatalities on urban and rural highways.

Although the RDG provided a more realistic approach than the application of a single lateral distance, it provides only a general approximation of the needed clear zone distance. Clear zone recommendations can be extrapolated for design speeds greater than the maximum ranges shown in the 4th Edition of the RDG, corresponding to 65 to 70 mph (100 to 110 km/h) and for Average Daily Traffic (ADT) greater than 6,000 vehicles/day or more. However, it is unclear if extrapolated values are optimized for posted speed limits (PSLs) greater than 70 mph (110 km/h) or for roads with ADTs significantly higher than 6,000 vehicle/day. Two of the key factors in assessing risk are design speed and traffic volumes. In some locations in the U.S., posted speed limits (PSLs) have been increased to 80 mph (129 km/h) or more, and there are many segments of highway in which the ADT is greater than 50,000 vehicles/day. Limited data has been collected to evaluate the effectiveness of clear zone recommendations which are not consistent with the existing ranges shown in the RDG. The width of the clear zone should be based on actual risks. Therefore, there is a need to analyze crash data to determine if revisions to the RDG clear zones are warranted to accommodate increased design speeds, locations with higher crash frequencies, and increased traffic volumes.

The results of this research could increase the safety of the traveling public and aid roadside safety design practitioners and transportation agencies in determining safer and cost-effective clear zone. The guidance developed through this research directly correlates with TCRS’ Strategic Plan and the number one goal in the TCRS mission statement which is, “develop, implement, and maintain guidance which will reduce fatal and incapacitating-injury roadway departure crashes.” Results of this research will help guide futures editions of MASH, the Roadside Design Guide, and other AASHTO documents.

Literature Search Summary

NCHRP Project 17-11, Determination of Safe/Cost Effective Roadside Slopes and Associated Clear Distance, developed relationships between recovery-area distance roadside features, vehicle factors, encroachment parameters, and traffic conditions for a range of highway functional classes and design speeds. The project developed baseline relationships for different functional classes of highways. These relationships are then adjusted for shoulder width, horizontal curvature, and ditch configuration. In NCHRP Project 17-11(03), Development of Clear Recovery Area Guidelines, the objective is to develop guidelines for roadside clear zones that can be incorporated into the AASHTO Roadside Design Guide. This is the final stage of work begun in 17-11 and continued in 17-11(02).

In the study, Benchmark Compliance Evaluation of the Roadside Design in an Urban, High-Density Area,

a roadside design compliance evaluation was performed in an urban, high-density area. Visits were made to over 100 single vehicle run-off-road (SVROR) injury-crash locations. Almost all locations were found to be non-compliant. A lack of the minimum recommended clear zone provision was the main cause of non-compliance, while 80% of all locations suffered from barrier misplacement. The study concluded that roadside design guidelines have been poorly implemented in the area studied, and findings indicate that more focus on proper, on-site design implementation is warranted. The authors are not aware of previous studies that have investigated roadside design compliance in other parts of the world. Hence, the authors encourage researchers to replicate this study in their respective geographical areas of interest.

The clear zone concept, related to lateral distances measured from the roadway edge, is also strongly correlated with roadside barrier runout length and length-of-need (LON), which are longitudinal measurements measured from a shielded hazard and based on a vehicle’s trajectory from the point of departure. Vehicle trajectories were documented and reported by Hutchinson and Kennedy in 1966 and Cooper in 1980 corresponding to vehicle tracks in the median in Michigan and tracks on snow-covered roads in Canada, respectively. Sicking and Wolford revisited guardrail runout lengths in 1996 and observed that the Cooper data for vehicle trajectories was associated with a higher benefit-cost and reduced number of unnecessary crashes with roadside barriers, which was subsequently confirmed by Coon, Sicking, and Mak in Guardrail Run-Out Length Design Procedures Revisited. Although the evaluation of roadside barrier lengths-of-need are not part of this research effort, responding agencies are encouraged to collect data which subsequently could be used to evaluate barrier runout lengths on these roadways as well.

This proposed research will build upon the above research, with an emphasis on roads with posted speed limits, traffic volumes, and crash frequencies which greater than the RDG ranges. The project will attempt to determine how best to accommodate road conditions beyond what the RDG currently recommends for clear zones.

Research Objective

The research objective is to recommend guidelines for clear zone values corresponding to conditions with design speeds, traffic volumes, and crash frequencies which are in excess of thresholds recommended in the RDG. The results of this research study would be used to inform updates to future publications of the RDG and identify safe and cost-effective guidance for roads at extremes of typical operational conditions. Specific objectives include:

• Perform a literature review and summarize published findings of encroachment rates based on ADT, including roadway geometrical effects, with attention to divided vs undivided roads;

• Determine encroachment rates for high-ADT and high-design speed roads, and compare with published historical values;

• Identify the distribution of lateral encroachment distances for high-ADT and high-design speed roads during ROR crashes;

• Determine criteria, including thresholds, which can be used to identify regions with increased rates of roadside departure which may require special treatment or considerations; and

• Recommend updates for the RDG to accommodate higher ranges of traffic volumes and speed limits than are shown in Table 3-1, and warrants for special consideration for roads with departure rates in excess of a standardized threshold of departure rates.

Urgency and Potential Benefits

Providing an appropriate clear zone is critical for reducing severe crashes on the nation’s highway. There is a need to develop clear zone guidance based on actual crash data at higher speeds, higher traffic volumes and high-crash locations. This research will provide a measure of assurance in the clear zone application and an increase level of safety for the traveling public. State Transportation Agencies may utilize the results of this research for policy development for the clear zone. It is anticipated that the results of this research will be incorporated into the AASHTO Roadside Design Guide.

Because there is no clear recommendation in the RDG to address clear zones for extreme conditions, guidance between states and for different road conditions may vary widely. It is unclear if recommendations provided for lower-speed and lower-traffic volume roads remain applicable at higher speeds and traffic volumes. It is possible that as state DOTs consider increases in posted speed limits, higher frequencies of SVROR crashes occur and result in catastrophic injuries without refined guidance for these conditions. In the absence of this research, needless severe injury and fatal crashes will likely continue to occur due to the hazard posed by obstructions within the clear zone.

This research is ranked as the top priority out of 5 topics by the AASHTO Technical Committee on Roadside Safety. This research is endorsed by the AASHTO Committee on Design

Implementation Considerations

The results of this research will assist state and local standards engineers to update the design manuals to provide guidance on clear zone. With proper review, the AASHTO TCRS could assist implementation through adopting the research results into the future update of AASHTO Roadside Design Guide. Implementation could also be supported through a discussion of the findings at a TRB AKD20 meeting and the conduct of the TRB webinar.

Recommended Research Funding and Research Period

Research Funding:

Funding for this research study is recommended at $450,000.

Research Period:

The research period for this project should be 36 months, completed in two consecutive phases. The 36-month evaluation should include 1 month following the execution of the contract resulting in a kickoff meeting with the panel, 1 month to review the interim report at the completion of Phase 1 and to evaluate and approve the revised research plan to address Phase 2, and a 90-day review period at the end of the contract to review the final report and recommendations, discuss conclusions with the panel, and recommend revisions to the contractor. As a result, the effective research period will span 31 months.

Problem Statement Author(S): For each author, provide their name, affiliation, email address and phone.

Hung Tang, P.E., New Jersey Department of Transportation, AASHTO Technical Committee on Roadside Safety, (609) 963-1839, hung.tang@dot.nj.gov

Andy Casey, P.E., Georgia Department of Transportation, AASHTO Technical Committee on Roadside Safety,

(404) 631-1700, acasey@dot.ga.gov

Cody Stolle, Midwest Roadside Safety Facility at the University of Nebraska-Lincoln,

(402) 472-4233, cstolle2@unl.edu

Potential Panel Members: For each panel member, provide their name, affiliation, email address and phone.

Hung Tang, P.E., New Jersey Department of Transportation, AASHTO Technical Committee on Roadside Safety, (609) 963-1839, hung.tang@dot.nj.gov

Person Submitting The Problem Statement: Name, affiliation, email address and phone.

Kristin Schuster, P.E.

Michigan Department of Transportation

Chair, AASHTO Technical Committee on Roadside Safety

Phone: 517-315-4421

Email: schusterk@michigan.gov