Safety Impacts of Intersection Sight Distance
III. RESEARCH PROBLEM STATEMENT
The provision of appropriate intersection sight distance (ISD) is considered a controlling element in intersection design. It is essential for safe vehicle maneuvers. The safety impact of providing or updating ISD, however, has not been properly analyzed or quantified. A research project that determines the safety impact of this critical intersection design element, with currently accepted statistical analysis procedures, is needed. The quantification of these safety impacts would allow alternative design decision-making evaluations and cost-benefit calculations that include changes to ISD. It would also provide the opportunity to evaluate the potential safety impacts related to providing or updating ISD to the distances proposed in NCHRP Report 383: Intersection Sight Distance and subsequently incorporated into the AASHTO A Policy on Geometric Design of Highways and Streets (Green Book) (1, 2). The need to fill this gap in the state-of-the-knowledge related to intersection design will become clearer with the release of the Highway Safety Manual in 2010 and the expectation for research-based alternative design safety analysis increases. The information produced by this study could be incorporated into design alternative comparison processes throughout the country.
IV. LITERATURE SEARCH SUMMARY
A review of the Transportation Research Board TRIS and Research in Progress databases revealed that very few projects have considered the quantification of ISD safety impacts. It was concluded that this subject had not been evaluated to currently accepted standards for safety data analysis.
In 1996, a new approach was proposed to determine ISD (1). This methodology was based on gap-acceptance and included in NCHRP Report 383: Intersection Distance (1). It also included the ability to more easily calculate ISD for both passenger cars and trucks (i.e., it allowed the selection of an ISD “design vehicle”). This approach to the determination of ISD is in the current AASHTO Green Book (2). A comparison of the ISD calculated with the new approach with those previously used and applied at many existing intersections reveals some differences.
A review of the adequacy in the state-of-the-knowledge related to the ISD safety impacts was completed during the development of the FHWA Interactive Highway Design Model (IHSDM). The results of this expert panel review were published in 2000 (3). It was found that no single research project was clearly the “most credible” of those available (3). Research results from Kulmala, Brude and Larsson, and Elvik were used to propose a crash (the term accident was actually used at that time) modification factor (CMF) that applied equally to each “corrected” intersection quadrant (4, 5, 6). Recently, however, this CMF was removed from the IHSDM because it was no longer considered to be acceptable by current standards of safety analysis.
The estimated safety impacts of ISD, based on older safety data analysis standards, continue to be used. For example, crash reduction factors from two documents were recently included within September 2008 Desktop Reference for Crash Reduction Factors (7). The first set of crash reduction factors appear to be based on the expert panel review noted in the previous paragraph (3). The second set of crash reduction factors is based on a national annual report from 1996 that focused on the impacts of highway safety improvements (7, 8). A 56 percent reduction in fatal collisions due to “sight distance improvements” is cited in addition to a 37 percent reduction in injury collisions (7, 8). The basis for these reductions and what is included in a “sight distance improvements” appears to vary and needs to be evaluated in more detail (8).
There was also at least one older safety analysis that considered the impact of ISD. This research was completed by David and Norman for the FHWA in the 1970s (9). In an urban area they calculated expected crashes per intersection per year as the intersection sight distance increased (9). A table was provided for a range of average daily traffic (ADT) and generally showed increases in crash reductions as ADT and the side approach ISD grew (9). This specific of this research should be evaluated in more detail.
Other research projects may have also indirectly considered the addition of intersection sight distance. This may have occurred, for example, during research projects that examine the addition of an offset turn lane or the reconstruction of traditional turn lane to an offset turn lane (10, 11). It is possible that the crash reduction results from these studies could be used in some manner to quantify the potential safety impacts of changes in particular types of ISD.
V. RESEARCH OBJECTIVE
The objective of this research is to produce a crash modification factor/function for the design decisions related to the provision of or changes to ISD. The product produced by this project should be helpful to the transportation professional and allow the crash impact and cost-benefit comparison of existing and/or proposed ISD designs. The results of the project should be developed in a manner that allows it to be directly used within the IHSDM and future additions or addendums to the Highway Safety Manual and/or the AASHTO Green Book. Some of the tasks that could be completed in this project proposal include:
· Completion of a comprehensive literature review and summary focused on the safety impacts related to the ISD needed for all intersection maneuvers. Some of this research might focus on increases to ISD that are a result of changes in intersection geometrics (e.g., the addition of an offset left-turn lane) and/or roadside maintenance. ISD needs at roundabouts and for different types of drivers (i.e., older drivers) should also be considered.
· Review of the existing ISD criteria and its adequacy. This review should include a comparison of all the criteria needed to calculate ISD (using the current policies) to current fleet and traffic flow characteristics.
· Collect appropriate data for an evaluation of the level of safety at intersections with varying ISD. The sites for this data collection will need to be properly considered and defined to meet the project objectives.
· Completion of appropriate statistical analyses that meet currently accepted standards for safety data evaluation. The results of this task should include crash modification factor/function(s) for possible incorporation into the ISHDM and the Highway Safety Manual.
· Propose new text based on the results of the research project for the next editions of the AASHTO Green Book, IHSDM, and/or Highway Safety Manual. The results of this project should also be used to update existing crash reduction factor documentation.
VI. ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD
VII. URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
The provision of ISD is a critical intersection design element. It is also considered a cost effective safety improvement based on the current state-of-the-knowledge. Unfortunately, the safety impacts of this improvement have not been quantified in a manner that is currently acceptable by today’s standard for safety data analysis. In other words, it is still not officially defined as a “proven” safety improvement countermeasure. A more comprehensive and robust research project is needed to properly fill this critical gap in the state-of-the-knowledge related to intersection safety. The need to fill this gap will become more obvious with the release of the Highway Safety Manual. This research project will fill this gap in the state-of-the-knowledge and also provide the ability to more properly defend decisions related to ISD (including cost-benefit analyses that incorporate the expected safety impacts of these decisions). The report from this project should include results that can be implemented through their inclusion within upcoming editions or addendums to the Highway Safety Manual, IHSDM, and/or the AASHTO Green Book (2).
1. Harwood, D.W., J.M. Mason, and R. Brydia. NCHRP Report 383: Intersection Sight Distance. Transportation Research Board, National Research Council, Washington, D.C., 1996.
2. American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, D.C., 2004.
3. Harwood, D.W., F.M. Council, E. Hauer, W. E. Hughes, and A. Vogt. Prediction of the Expected Safety Performance of Rural Two-Lane Highways. FHWA-RD-99-207. FHWA, Turner–Fairbank Highway Research Center, McLean, VA., 2000.
4. Kulmala, R. Safety at Three- and Four-Arm Junctions: Development and Application of AccidentPrediction Models. VTT Publication 233, Technical Research Centre of Finland, Espoo, 1995.
5. Brüde, U., and J. Larsson. Countermeasures at Junctions Taken As Part of the Regional Road Authorities’ Traffic Safety Program. VTI-Rapport 292, Swedish Road and Traffic Research Institute,Linköping, 1985.
6. Elvik, R. Meta Analysis of Intersection Accidents, Institute of Transport Economics, Oslo, Norway.
7. Behar, G., M. Masliah, and R. Wolff, and P. Park. Desktop Reference for Crash Reduction Factors. FHWA-SA-08-011. FHWA, Office of Safety, Washington, D.C., September 2008.
8. Federal Highway Administration. The 1996 Annual Report on Highway Safety Improvement Programs. FHWA-SA-96-040. FHWA, Washington, D.C., 1996.
9. David, N.A. and J.R. Norman. Motor Vehicle Accidents in Relation to Geometric and Traffic Features of Highway Intersections: Volume 11 – Research Report. FHWA-RD-76-129. FHWA, Washington, D.C., July 1979.
10. Persaud, B., C. Lyons, K. Eccles, N. Lefler, and F. Gross. Safety Evaluation of Offset Improvements for Left-Turn Lanes. FHWA-HRT-09-035. FHWA, Washington, D.C., June 2009.
11. Hochstein, J.L., T.H. Maze, T.M. Welch, H. Preston, and R. Storm. Safety Effects of Offset Right-Turn Lanes at Rural Expressways. In the Proceeding of the 2007 Mid-Continent Transportation Research Symposium, Iowa State University Ames, IA, August 2007.