Abstract:
At the first Urban Street Symposium in June 1999 the feasibility of converting urban four-lane undivided roadways to three-lane cross sections was introduced. Several successful examples of this type of conversion were discussed. It was found that in some cases this type of conversion might be able to improve safety with only a small reduction in operations. A significant amount of work has been done related to the potential safety and operational impacts of four-lane undivided to three-lane conversions since 1999. This paper summarizes the content of some guidelines and research completed by the authors. Data from case study conversions are presented and feasibility determination factors described. A CORridor SIMulation (CORSIM) software package sensitivity analysis approach was used in two projects to support the discussion of the factors related to the traffic flow differences of similar four-lane undivided and three-lane roadways. The variables considered in the analyses were total entering traffic volume (up to 1,150 vehicles per hour per direction), and different levels of left-turn traffic, access point densities, percent heavy vehicles, and bus stop activities (e.g., bus dwell times and headways). Investigations of the difference in signalized side-street vehicle delays and off-peak average arterial travel speed were also completed. The results of all the work recently completed in this area should help urban street designers decide whether a four-lane undivided to three-lane conversion is feasible at a particular location, and whether it will help improve their situation.
