One of the most common reasons for continuing a traffic stop is the smell of marijuana. The usual scenario is that you are pulled over for a minor traffic violation. As he's walking up to the car, or while talking to you and getting your information he claims to smell marijuana. That will give them cause to investigate further, which usually ends up in a search of your car.
The obvious problem with a search based on the smell of marijuana is that there is no way to prove the officer didn't smell it; which means an officer can make it up and you have no way to prove it. Even if they didn't find marijuana they will claim that it must have been in the car earlier. Of course if they don't find anything you probably aren't going to be in court anyway.
The Washington recently had an article by Radley Balko titled "The drug war exception to the Fourth Amendment". As the title suggests, Balko suggests that Courts are condoning tactics used in the so called "war on drugs", which is eviscerating the Fourth Amendment. Probably the best examples of that are cases where the stop is based on the officer's claim that they can smell marijuana from a moving vehicle. As absurd as that sounds, for the most part courts are buying it, and refusing to suppress evidence. The potential for abuse should be obvious; if you look suspicious - or fit a profile - you can be stopped because the officers says he smelled marijuana as he drove by and when that happens how can you prove the officer was lying?
The obvious solution to this is extra scrutiny - perhaps even using a little common sense. As Balko shows though, that isn't successful nearly as often as it should be. Most judges don't like to grant motions to suppress, because the result is that the case is usually dismissed. The courts are the gatekeepers, and are in charge of enforcing the constitution. It's up to defense lawyers to keep raising the issue, and try to prevent any further erosion of the Fourth amendment.
If you want more information on your rights during a traffic stop check out our article here.