There were some tests in the UK for some on-the-spot detectors, but I seem to recall they didn't work accurately enough.
Turambar has a point that most stoned people tend to drive pretty slowly, but I suppose my concern is concentration, it's very hard to focus on things when you have smoked cannabis, or rather, too easy to be distracted by something else, I don't want to be mown down by a car doing 12 miles an hour because the guy got hypnotized by his own dashboard

Other than that, considering that pot is considerably less dangerous than Tobacoo or Alcohol, I don't really have a problem.