But what about their propensity to drive really slow?
You mean like a bicycle, moped, a new parent driving extra slow, an elderly driver, or any other street legal slow vehicle does?
I understand that drivers assume the speed LIMIT is a speed suggestion, and experience road rage when driving under the limit, but there is nothing -- nothing -- illegal or unsafe about staying under the speed limit.
If there is something unsafe about it, there should be a minimum posted speed (many places have this).
As a US citizen I will tell you this is false. We have basically no national traffic laws.
Your local state, city or other local government may have passed a law like this.
For example, in my area, it was made illegal to stay in the left lane moving more slowly than those behind you -- you must yield to the right if they want to go more quickly.
But some 99% of Americans do not live under this local law I'm discussing.
http://codes.findlaw.com/tx/transportation-code/transp-sect-...
Trivial to argue that reduced speed is required for safe operation in this case, the research in this very article may be useful in that defense
Also, a Google Self-driving car got pulled over the other day for simply going the speed limit, it did not get a ticket but I thought I would mention it.
It's generally wise not to do anything other drivers will find surprising and I guess 35mph on a motorway falls in that category.
Most important thing when driving is to be predictable. It's predictable to be traveling plus or minus 10 mph from what is posted.
It isn't predicted to be traveling 30mph+ below the speed limit.
(no comment on being high, just saying, it's super dangerous to be doing 30-40 on a 65mph road).
Go to any motorcycle riding school and all their instructors will teach you that riding slower than the traffic around you is more dangerous than maintaining the average traffic speed, even when above the legal speed limit.
I don't even think that pot makes people drive slowly. IMO it's a result of stoned people being afraid of giving the police any excuse to pull them over.
Samuel C. Tignor and Davey Warren. "Driver Speed Behavior on U.S. Streets and Highways." Institute of Transportation Engineers: 1990 Compendium of Technical Papers, 1990 August, p. 85. "The accident involvement rates on streets and highways in urban areas was highest for the slowest 5 percent of traffic, lowest for traffic in the 30 to 95 percentile range and increased for the fastest 5 percent of traffic."
Slowing down is safer because it significantly reduces the severity of the traffic accidents that do happen.