The Interactive Highway Safety Design Model: designing for safety by analyzing road geometrics

Public Roads, Summer, 1994 by Jerry A. Reagan

Background: Safety and Geometric Design

The late 1980s saw a renewed interest in safety and geometric design in the United States. The Transportation Research Board (TRB) Committees on Geometric Design and the Operational Effects of Geometric Design launched a five-year series of sessions, beginning in 1988, on the state of the practice of five geometric design topics: sight distance, interchanges, intersections, alignment, and cross sections.(1-3) These sessions were held at the annual TRB meetings in Washington, D.C. At the state level, interest in geometric design was evidenced by a broad range of research problem statements submitted to and funded under the National Cooperative Highway Research Program (NCHRP).

Meanwhile, in March 1988, Federal Highway Administration (FHWA) management designated Highway Safety Design Practices and Criteria as a high-priority research and development (R&D) area. The objective of the program is to develop an integrated design process that systematically considers both the roadway and the roadside in developing cost-effective highway design alternatives. This process will help the highway designer evaluate design alternatives from a safety standpoint. Moreover, in developing the process, objective measure(s) of highway safety will be established.

Figure 1 shows the initial concept of the components of the integrated design process. The researchers developing the program envisioned that this process would begin with a design alternative developed by the highway designer in accordance with agency guidelines. This alternative would be checked for potential safety problems against safety data in each of four modules of a computer system. These modules would be a roadway module (which would essentially cover multivehicle accidents); a roadside module (which would essentially cover single-vehicle accidents); a consistency module (which would be based on speed profiles, since large changes in speed between successive roadway sections are believed to contribute to accidents); and a physics module (which would measure speeds and lateral accelerations based on a computer simulation of the interaction between the vehicle and the roadway). The design would be checked sequentially through these four modules. The designer would have to decide how to solve any potential safety problems identified through the process.

IHSDM Defined

The program's first product was a six-volume synthesis on highway safety research.(4) This study specifically addressed access control, alignment, cross sections, interchanges, intersections, and pedestrians and bicyclists--topics selected based on recommendations from TRB and earlier synthesis studies.(5,6)

These studies pointed out several broad issues that would have to be addressed if the effort to develop an integrated design process was to succeed:

* The effort should integrate all safety research related to geometric design into a usable form for the designer. Past research only dealt with specific problems and gave little thought as to how results would be incorporated into the design process. For example, the synthesis study has data on the relationship between safety (accidents) and geometric features (medians, grades, etc.). The volume on intersections indicates that "intersections with poor sight distance experience higher accident rates." Unfortunately, "poor" sight distance is not quantified.

* The designer should be able to correct problems as the design was being reviewed. The process should be interactive.

* Standard definitions should be developed and adhered to for study variables. There were no common definitions for the variables used in the older studies reviewed; thus, the results of the different studies could not be combined. Some studies dealing with the same problem, such as the safeness of painted pedestrian crosswalks, reached different and opposite conclusions.

* Correct statistical procedures should be followed. Many of the early studies arrived at conclusions that were not supportable due to small sample sizes and questionable analysis.

* Measures other than accidents for evaluating highway safety should be pursued. Historically, attempts to relate accidents and geometric design features have not been successful; the correlation between accidents and geometric design features was very weak. Accidents might not be the best measure of a highway's safety.

These issues clarified the concept of what is now known as the Interactive Highway Safety Design Model (IHSDM). Although the model's exact format was unknown, its purpose was and remains clear. IHSDM will provide information on safety and geometrics in a format that a highway designer can use. It will guide the designer in evaluating the safety of the design.

IHSDM

The Geometric design process varies considerable among the 50 states. However, it can generally be divided into two phases preliminary design often associated with the preparation of environmental impact statements (EIS) and detail design associated with the preparation of plans, specifications, and estimates (PS&E). Because of the type of data available at each phase, two versions of the IHSDM would be needed.