In the nation of Partoria, large trucks currently have a much higher rate of traffic accidents per mile driven than other vehicles do  However, the very largest trucks-those with three trailers-had less than a third of the accident rate of single- and double-trailer trucks
Clearly, ttherefore, one way for Partoria to reduce the number of traffic accidents would be to require shippers to increase their use of triple-trailer trucks
No matter what changes Partoria makes in the regulation of trucking, it will have to keep some smaller roads off-limits to all large trucks.