CTRE is an Iowa State University center, administered by the Institute for Transportation.

Address: 2711 S. Loop Drive, Suite 4700, Ames, IA 50010-8664

Phone: 515-294-8103
FAX: 515-294-0467

Website: www.ctre.iastate.edu/

Iowa State University--Becoming the Best

System-Wide Safety Treatments and Design Guidance for J-Turns


Principal investigator:

Co-principal investigators:

Project status


Start date: 09/01/14
End date: 09/30/16


Report: System-Wide Safety Treatments and Design Guidance for J-Turns (4.21 mb pdf) January 2017



About the research


In support of its Toward Zero Deaths (TZD) initiative, the Missouri Department of Transportation (MoDOT) initiated this research project to develop guidance on treatments that can reduce crashes and fatalities. The first task for this project was to synthesize the literature and state of the practice related to system-wide safety treatments and document the treatments' effectiveness. In particular, the objective was to examine those treatments that have not already been implemented in Missouri. The safety effectiveness, implementation guidelines, limitations, costs, and concerns of the treatments were documented. The identified safety treatments work in conjunction with the “Necessary Nine” strategies identified in the Missouri Blueprint. Accordingly, the synthesis covered three areas: (1) horizontal curves, (2) intersections, and 3) wrong-way crashes. The reviewed treatments included signing, geometric design and access management, intelligent transportation systems (ITS), pavement markings, and signal control enhancements to improve safety. In the last few years, MoDOT has replaced several high-crash intersections on rural highways in the state with J-turns. Given their safety effectiveness and low cost, J-turns have become a preferred alternative to replace high-crash two-way stop-controlled intersections on high-speed highways. Unfortunately, national guidance on the design of J-turns is very limited. This project addressed this gap in practice by developing guidance on spacing and acceleration lanes. A thorough examination of crashes that occurred at 12 existing J-turn sites in Missouri was conducted. The crash review revealed the proportions of five crash types occurring at J-turn sites: (1) major road sideswipe (31.6%), (2) major road rear-end (28.1%), (3) minor road rear-end (15.8%), (4) loss of control (14%), and (5) merging from U-turn (10.5%). The crash rates for both sideswipe and rear-end crashes decreased with an increase in the spacing from the minor road to the U-turn; J-turns with a spacing of 1,500 ft or greater experienced the lowest crash rates. A calibrated simulation model was used to study various volume scenarios and design variables. For all scenarios, the presence of an acceleration lane resulted in significantly fewer conflicts. Therefore, acceleration lanes are recommended for all J-turn designs, including lower volume sites. Moreover, while a spacing between 1,000 and 2,000 ft was found to be sufficient for low-volume combinations, a spacing of 2,000 ft is recommended for medium- to high-volume conditions.