Statistics indicate that red-light running has become a significant safety problem throughout the United States. There is a wide range of potential countermeasures to the problem of red-light running; an increase in yellow duration is one countermeasure. The objective of this research was to quantify the effect of a change in yellow-interval duration on the frequency of red-light violations. A before-after study is described and the resulting data are used to quantify the effect of increasing the yellow interval on the frequency of red-light violations. Based on this research, it was concluded that (a) an increase of 1.0 s in yellow duration (such that it does not exceed 5.5 s) will decrease the frequency of red-light violations by at least 50%; (b) drivers do adapt to the increase in yellow duration, but this adaptation does not undo the benefit of an increase in yellow duration; and (c) increasing a yellow interval that is shorter than the value obtained from a proposed recommended equation published by the Institute of Transportation Engineers is likely to yield the greatest return (in terms of a reduced number of red-light violations) relative to the cost of retiming a yellow interval in the field.