Sites with high or low crash rates are likely to move towards the mean as a matter of course, even if nothing changes.

Locations are selected by looking for sites with higher than average crash rates, and to address citizen complaints.

The crash rate returned to its former level after people became familiar with the new arrangement.

Drivers from ages 16 to 19 have the highest crash rate.

"You get those two factors together, and they make for higher crash rates."

However, this does not translate to significantly lower crash rates in Ireland.

Single-vehicle crash rates for passenger cars fell by a third.

The earlier study found a 34 percent reduction in fatal crash rates.

What is particularly striking, however, is how rapidly the crash rate drops with age.

In one study, the highest crash rate occurred during the first month after teenagers got their license.