Compact roundabouts as a solution for semi-rural two-way stop-controlled intersections with high crash severity

Steven Diebol

Senior Engineer
Concord Engineering

Steve is a 2005 graduate of Michigan State University with 16 years of experience with consulting firms in the transportation engineering field. He is a licensed Professional Engineer in three states, a Professional Traffic Operations Engineer, and a Road Safety Professional. His experience includes intersection and roadway traffic modeling, safety studies, traffic signal and illumination design, and roundabout analysis and design. His roundabout experience includes performing operational and safety analyses of existing and proposed roundabouts, roundabout microsimulation analysis, roundabout safety studies, peer review of roundabout designs, managing roundabout design projects, and assisting research teams on developing design practices for accommodating trucks at roundabouts. Steve is a Senior Engineer for Concord Engineering in Bellevue, Washington where he lives with his wife Julia, a safety consultant, and their 7-year-old triplet sons.

The Lenawee County Road Commission in Lenawee County, Michigan sought solutions to a common problem of severe crashes occurring at two-way stop-controlled intersections. Typical sites involved a semi-rural setting, high speeds (>45mph) on at least one roadway, and sight distance issues or poor alignment. Preliminary engineering studies of two locations revealed signal warrants (volumes and crashes) were not met and all-way stop control was not warranted and would impede traffic flow. Many crashes were attributed to drivers failing to yield to traffic on the uncontrolled approaches. Short-term low-cost countermeasures were tested at one location. These were primarily additional signage, pavement markings, and enhancing visibility of existing traffic control devices. Three years after implementation of the short-term low-cost countermeasures the same crash patterns continued and severity did not decrease at the one test location. The county next considered a compact roundabout as a long-term improvement for two main reasons: 1) the potential reduction in crash severity due to the change in traffic control and intersection geometry and 2) less impacts and cost with a smaller intersection footprint than other options. At the time of this abstract one compact roundabout has been designed (95’ ICD) and bid for construction, and two other locations are under consideration in the project development process. For the completed design, engineering judgement was used to assess design criteria to blend elements of traditional single-lane roundabouts with features of mini-roundabouts. Design challenges included controlling deceleration into roundabout approaches, accommodating large vehicles, and minimizing right-of-way and utility impacts.

Learning objective 1: Identifying short term and long term safety countermeasures for intersections
Learning objective 2: Targeting countermeasures for specific types of crash causes/contributing factors
Learning objective 3: Assessing performance of implemented countermeasures
Learning objective 4: Understanding compact roundabout design components blended from design criteria for single lane roundabouts and mini roundabouts

Target audience: Safety analysts dealing with similar high-speed semi-rural locations, roundabout designers seeking guidance on compact roundabout design practices, road agencies facing challenges of improving safety with limited budget

This presentation is a case study of how qualitative safety assessments and engineering analysis were used to recommend roundabouts as a long-term countermeasure to replace ineffective short-term improvements, considers the current state of practice of compact roundabout design when examining design elements blended from single lane/mini roundabout criteria, and provides discussion about design features of compact roundabouts that can provide construction cost savings.