Correlation is not causation

The Wall Street Journal has an opinion piece on speed limits: OpinionJournal – Featured Article which argues that raising the speed limit has been correlated with an increase in safety.


The article, citing some partisan research, implies that there is in fact either no relation with speed limits on interstates and safety, or that the increase is the cause of the reduction in injury and death.
This has been going around for awhile, so let’s think about causes:
(1) Higher speed limits on interstates attract traffic off of more dangerous lower quality roads. This increases overall safety (though may increase interstate crashes while decreasing non-interstate crashes). (Supports argument)
(2) This is a before and after study and there is a time trend. Over this same period, emergency response, and the ability of hospitals to save lives has improved. Air bags have beeen installed, so the crashes may in the end be less severe. Policies against DWI have changed. The correlation may be due to extraneous (non-speed limit) factors. (Counters argument about speed limits being factor)
(3) Vehicles that are traveling faster crash faster, and more dangerously. We assume speed limits are correlated with speeds. When crashes occur at higher speeds they are more dangerous than lower speed crashes. (Counters argument)
(4) Drivers traveling faster are more likely to lose control or be unable to respond in time to a sudden unexpected event. (Counters arguments)
(5) Vehicles that are traveling at different speeds are more likely to crash. Vehicles traveling the same speed in the same lane will by definition not crash. If higher speed limits reduce variance, this may reduce deaths/injuries/crashes (Supports argument).
My colleague Gary Davis has written about this. See this and this: Putting limits on the speed-versus-safety debate
Davis, G.A. 2002. Is the claim that ‘variance kills’ an ecological fallacy? Accident Analysis and Prevention, 34: 343-6
In short, the logic is a lot more complicated and ambiguous than the Wall Street Journal would lead its readers to believe. Crashes and injuries and fatalities may change for a variety of reasons. Higher interstate speed limits have off-setting effects that are not clearly disentangled.