Left-Turn Prohibition and Partial Grade Separation for Signalized Intersections: Planning-Level Assessment

Recurring congestion at signalized intersections is caused by the inability of traffic signal controls to serve demand even in cities where advanced traffic signal timing and management systems are in operation. Left-turn prohibition (LTP) and partial grade separation with low-clearance underpass (LCUP) are possible congestion mitigation actions for urban complex and congested signalized intersections that have exhausted other options for capacity enhancement, such as lane addition and signal timing optimization. LTP and LCUP improve traffic signal efficiency by eliminating signal phases, increasing green splits, or adding lanes. They also improve intersection safety by eliminating certain conflicts. At the same time, they necessitate traffic rerouting, may have negative impacts at downstream locations, and may have perceived or actual impacts to accessibility and neighborhood character. A planning-level assessment method to quantify and compare the benefits and costs LTP and LCUP was developed to examine the potential feasibility of these congestion mitigation actions prior to conducting, e.g., extended data collection, detailed analysis, simulation, evaluation, and public meetings. The assessment considers intersection delays, accident frequency, and cost-benefit analysis based on industry publications. The proposed method uses standard methodology and nationally accepted default values and conservative assumptions to expedite assessment by minimizing the number of required inputs. Due to the complexity of analysis for LTP and LCUP for various time periods and multiple locations, a spreadsheet-based tool was developed. A case study demonstrates the combined use of travel time, safety, and economic savings of the proposed method.