Road safety knowledge and the lack thereof : the U.S. perspective.

It is difficult to fiend out how good available estimates of effectiveness are. Generally, studies show with a reasonable certainty whether it is better to implement a countermeasure rather than to do nothing, but results are far less certain when trying to determine how much better it is for a specific situation. One can say there are few good estimates of the effectiveness of safety measures. This situation is, however, slowly improving.