One issue in urban planning is the unstated presumption that slower is always safer. Gattis' study results also indicate that speed may not be as significant a factor in urban street safety as is generally assumed. In examining accident rates on two adjacent, parallel through streets, Gattis found significant differences in accidents, but not speeds. Although one street was 50 percent wider than the other, speeds on these streets were essentially the same. However, accident rates on the narrower street were more than 50 percent higher and personal injury accidents were 25 percent higher than on the wider street.
Planners classify streets according to their intended use.
Local streets should be short and meant for residential traffic. They typically connect to collector streets, which funnel traffic out of neighborhood and toward arterial streets. Arterial streets are intended to move through traffic from one area to another with minimal impediment. Problems arise when these principles are misapplied. Streets designed for a lower level of traffic actually end up carrying a higher volume.