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Impact of Left-Turn Phasing on Older and Younger Drivers at High-Speed Signalized Intersections

Map of Iowa showing counties in different colors.

Percentage of drivers age 65 and older by county in Iowa


Principal investigators:

Student researcher:

Project status


Start date: 07/15/03
End date: 08/30/04


Report: Impact of Left-Turn Phasing on Older and Younger Drivers at High-Speed Signalized Intersections (2.4 mb pdf) August 2004


Sponsor(s): Iowa Department of Transportation

About the research

Abstract: Several recent studies have demonstrated differences in safety between different types of left-turn phasing--protected, permitted, and protected/permitted phasing. The issue in question is whether older and younger drivers are more affected by a particular type of left-turn phasing at high-speed signalized intersections and whether they are more likely to contribute to a left-turn related crash under a specific type of left-turn phasing.

This study evaluated the impact of different types of left-turn phasing on older and younger drivers at high-speed signalized intersections in Iowa. High-speed signalized intersections were of interest since oncoming speeds and appropriate gaps may be more difficult to judge for older drivers and those with less experience. A total of 101 intersections from various urban locations in Iowa with at least one intersecting roadway with a posted speed limit of 45 mph or higher were evaluated. Left-turn related crashes from 2001 to 2003 were evaluated. Left-turn crash rate and severity for young drivers (14- to 24-year-old), middle-age drivers (25- to 64-year-old), and older drivers (65 years and older) were calculated.

Poisson regression was used to analyze left-turn crash rates by age group and type of phasing. Overall, left-turn crash rates indicated that protected phasing is much safer than protected/permitted and permitted phasing. Protected/permitted phasing had the highest left-turn crash rates overall.