One of the methods police in the US use to determine find driver's who are stoned is they look for people who are driving UNDER the speed limit...
Along with other problems stoned drivers cause, this is definitely one of them. Back in my experimental days, I partook in the occasional joint. Late one evening while sharing a joint with a friend, I was driving along one of Vancouver's main streets.
Up ahead a traffic light went red so I stopped for it.
When it turned to green, I continued driving until it went red again, waited for it to turn green, and then started driving again.
It went red again, I stopped, it went green and I drove.
Again it turned red, again I stopped, again it turned green and finally I got past it.
I think at that point we realized what had happened and started laughing hysterically.
Yup, I stopped at least 4 times for the same red light, so we must have been driving sloooooooooooooooooooooow.
Good thing the streets were quiet that night.