"In an urban 60 km/h speed limit environment ... the relative risk of being involved in a casualty crash ... was found to approximately double for each 5 km/h increase in free traveling speed," Arrive Alive says on its site.There is even a relative risk formula used to determine the likelihood of a fatal crash. Using figures from case studies for various speeds above and below a speed limit threshold. Because people rarely just do things because they're told, authorities say that changing driver behaviour is best achieved via deterrents like penalties. Hence the enforcement of speed limits via cameras and fines. According to Arrive Alive, studies show a significant decrease in fatality risk when an area has speed enforcement.