[ARCHIVE ANSWER]
Experienced marijuana smokers can pass all roadside field sobriety tests. Experienced pot users can drive safely. If a cop thinks you are high, but you pass all the roadside tests, he should not arrest you. If he does, you can easily win in court.
According to F.A.R.S. (Fatal Accident Reporting System) in 2006, 111,000 drivers were seriously injured or killed while under the influence of alcohol while only 11 were seriously injured or killed from marijuana.
Anytime a citizen is involved in a traffic accident that causes serious bodily injury or death, the cops can legally have the driver's blood tested.
We still don't know if the 11 who tested positive for THC were high at the moment of impact. They could have smoked the weed days earlier and the THC was detected from that.