Current geometric design of roadway elements is based primarily on the design speed. The design speed is used by roadway engineers to design appropriate safety elements such as vertical and horizontal curves, stopping sight distance, and guardrail needs. On rural highways, operating speed is often higher than the design speed, which may become problematic from a safety standpoint. In rural settings, there are also more free-flowing traffic conditions with limited enforcement opportunities. These circumstances often lead to increased crashes.
With these issues in mind, the Illinois Department of Transportation (IDOT) initiated a special project on speed harmonization—a proactive strategy to improve traffic flow and safety based on prevailing or anticipated conditions. Specifically, ICT researchers analyzed how roadway design engineers can predict crash rates, operating speed profiles, and safety benefits based on roadway design elements.
The project was conducted under the direction of a Technical Review Panel chaired by Kyle Armstrong, Acting Engineer of Traffic Operations for IDOT. Yanfeng Ouyang, professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign, served as ICT principal investigator of the study. Ouyang’s primary research focus is transportation engineering.
To understand current practice and feasible strategies for speed harmonization, the ICT researchers conducted a comprehensive literature review and a series of interviews with Illinois county engineers.
Roadway design and maintenance practices and their impacts on Illinois roadway safety are summarized in the project’s final report, Speed Harmonization: Design Speed vs. Operating Speed (R27-SP30), published in October 2016. The final report also presents a new methodology that summarizes the relationship between design speed and operating speed, as well as the safety impacts of various geometric elements.
Ouyang says, “Our integrated modeling framework includes modules for geometric design simulation, operating speed‐profile prediction, and crash rate prediction. In addition, we developed a new methodological framework for a benefit-cost analysis to quantify the economic benefits of various strategies for roadway safety improvement.”
Armstrong adds, “The implementable outcome of this project for IDOT and local agencies is an Excel VBA-based application into which designers can input roadway parameters along with budget constraints to aid in selecting appropriate design elements and safety countermeasures in an effort to reduce the number and severity of crashes.”
The graphical user interface of the application efficiently helps an agency visualize the safety impact and benefits and costs of countermeasures. The input form for predicting operating speeds is shown in the figure below (top). A user can enter inputs such as traffic condition, design speed, and the basic geometric design requirements. The software can simulate the geometric design, predict operating speed distribution, and assess the expected crash rates based on these inputs. The countermeasures evaluation input screen is shown on the bottom.
Once the simulation is completed, the results of the safety evaluation and economic analysis are presented. The results include expected crash rates, speed consistency level, and benefit-cost ratios for each safety improvement strategy. The two figures below are screenshots of sample outputs for widening an existing shoulder to different widths.