Driving under the influence is a criminal offense and usually it is easy to spot the driver who is intoxicated. This seems not to be the case with stoned drivers. In a 2012 study published in The Journal Psycho Pharmacology, only 30% of people under the influence of THC failed the field test. The ability to tell if the driver is affected while driving  stoned really depends on how accustomed he/she is to being medicated.  As more states legalize medicinal use and recreational use,  the huge question will be how much is to0 much and how does it compare to driving while under the influence of alcohol.

http://www.nytimes.com/2014/02/18/health/driving-under-the-influence-of-marijuana.html?_r=0

I think this will be a difficult problem to tackle because the affects of marijuana and alcohol are totally different. Tolerance will play a big role because experienced  pot smokers will have no problem starting up their car and taking a trip somewhere.  The real problem starts when new marijuana smokers with low tolerance drive.  The affects of marijuana will be strong and new users will be impaired.  I feel it is up to the person to make the right decision and be responsible but then again there will always be a handful of people who make mistakes.