Recent information from the Marijuana Policy Project (MPP) addresses the problem of determining whether a driver is under the influence of marijuana, or whether the marijuana in his or her system is days, or even weeks, old.  Blood testing is the most reliable method of detecting THC levels, but even those tests could lead to the conviction of sober drivers.

Here in Fairfax County, Virginia, DUI drug cases are called "DUID."  This may well be the future of DWI laws.  They are moving towards making cars that won't start if the driver has alcohol on his or her breath, and so eventually they would have to resort to prosecuting more DUID cases in order to keep the money flowing.  (Drivers under the influence of alcohol wouldn't be driving if their car won't start, resulting in fewer DWI charges, resulting in less money for the County.)

In practical terms, most cops don't automatically think "DWI" when they smell marijuana during a traffic stop.  They will usually search the vehicle, find the marijuana, and charge the driver with possession of marijuana.  If there has been an accident, or if the driver is obviously intoxicated, then they might start doing DWI tests.

However, most cops don't realize that the Standard Field Sobriety Tests are designed for alcohol, and they are not automatically "just as good" for investigating other types of cases.

So, if the driver doesn't do so well on field sobriety tests, then the cop will probably make an arrest, and then the driver will have his or her blood drawn.  For marijuana, there is no set "legal limit" like there is for alcohol and a few other drugs.  In Virginia, there are a few drugs that do have set limits, and they are listed right here:

(a) 0.02 milligrams of cocaine per liter of blood,
(b) 0.1 milligrams of methamphetamine per liter of blood, 
(c)  0.01 milligrams of phencyclidine per liter of blood, or
(d)  0.1 milligrams of 3,4-methylenedioxymethamphetamine per liter of blood.

For all other drugs, there is no set amount, so you should expect that a good Fairfax County DWI Attorney is going to make it very difficult for the State to prove such a charge.