"Practitioners generally believe that additional capacity afforded by additional lanes is associated with more safety. How much safety and for what time period are generally not considered. Comparison of SPFs of multilane freeways suggests that adding lanes may initially result in a temporary safety improvement that disappears as congestion increases. As annual average daily traffic increases, the slope of SPF, described by its first derivative, becomes steeper, reflecting that accidents are increasing at a faster rate than would be expected from a freeway with fewer lanes."

Yikes. I put in a request for the full document so I can take a look at it and see if there are any juicy bits to use against freeways :)

SPF stands for safety performance function, by the way. The study was conducted in Colorado, California and Texas.

I've heard about one study that explored the road improvement paradox: better roads don't reduce commute times, but instead encourage longer drives. The study would seem to recommend that making roads *worse* -- making them narrower, taking away lanes, etc. -- would achieve many urban goals more effectively.