About the research
Four-lane undivided roadways in urban areas can experience a degradation of service and/or safety as traffic volumes increase. It has been found that the conversion of a four-lane undivided cross section to three lanes (i.e., one lane in each direction and a two-way left-turn lane) can improve safety and maintain an acceptable level of service. These guidelines summarize the results of past research in this area and qualitative/quantitative before-and-after safety and operational impacts of case study conversions located throughout the United States and Iowa.
The simulation results, along with past research and case study results, appear to support following volume-related feasibility suggestions for four-lane undivided to three-lane cross section conversions. It is recommended that a four-lane undivided to three-lane conversion be considered as a feasible (with respect to volume only) option when bi-directional peak-hour volumes are less than 1,500 vehicles per hour, but that some caution begin to be exercised when the roadway has a bi-directional peak-hour volume between 1,500 and 1,750 vehicles per hour. At and above 1,750 vehicles per hour, the simulation indicated a reduction in arterial level of service. Therefore, at least in Iowa, the feasibility of a four-lane undivided to three-lane conversion should be questioned and/or considered much more closely when a roadway has (or is expected to have) a peak-hour volume of more than 1,750 vehicles.
This feasibility, however, must be determined by an evaluation of the factors identified in the guidelines in this report (along with any others that may be relevant for a individual corridor). The expected benefits, costs, and overall impacts of a four-lane undivided to three-lane conversion should then be compared to the impacts of other feasible alternatives (e.g., adding a raised median) at a particular location.