Effect of Roadway Lighting on Safety in Utah

A significant portion of severe crashes and severe crashes involving vulnerable road users, 29 percent and 39 percent respectively, occur at night in Utah, despite roughly only 25 percent of traffic occurring at night. Nighttime crash causes are less understood than daytime crashes as there are fewer nighttime crashes, but it is generally assumed that increasing street lighting can help mitigate these crashes. To test this assumption, the Utah Department of Transportation was interested in developing Crash Modification Factors (CMFs) specific to the state of Utah which correlate the quantity and quality of light to crash outcomes, to observe how specific light quantities and qualities might reduce nighttime crashes. Average illuminance and light uniformity metrics were measured for several arterial roads in Utah and statistical models were used to build models for estimating CMFs. CMFs indicated that average illuminance was not correlated to a significant change in crashes except when increasing illuminance above 3.2 lux. This indicates that some light is better than none, but the benefit of increasing existing light levels may not be significant. However, CMFs for the uniformity of light indicated that less uniformity correlated to fewer crashes at lower average illuminance, and more crashes at higher illuminance. This may show that it is more important to light specific locations than to light entire road segments evenly, unless light levels are already high. Despite the possible benefit of increasing uniformity on roadways with high light levels, most uniformity CMFs suggested that future research should be focused on locations like crosswalks where lighting is most important.