A main  traffic  safety  priority in  many  countries  is  implementing measures  that reduce  road  user  speed  and increase  speed limit compliance. This study compares changes in the  speed distribution  and estimated accident risk brought about by three measures:

1. speed limit change from 110 to 100 km/h on roads without speed cameras
2. speed  limit  change  from  90  to  80  km/h  on  roads  with  speed  cameras
3. introducing new  speed  cameras  on roads  with a  speed  limit  of 90  km/h

The  accident  risk  is  estimated  from  mean  speed  since  the  impacts of  a change in mean speed on the number of accidents and of injured and killed people are well known. The results indicate that installing speed  cameras  can  reduce  average  speed  more  than can reducing the  speed  limit  by  10 km/h, at least at the camera locations. Moreover, speed cameras reduce the standard deviation of speed and the percentage of drivers exceeding the speed limit by far more than does lowering the mean speed. This implies that the  relationship  between  change  in  speed  distribution,  not  only  mean  speed,  and  traffic  safety merits further investigation.