Don't drive under the influence of anything stronger than moderate doses of caffeine. If you want to be able to safely drive, I think you should avoid any inebriating substance. Sceletium tortuosum as ragabr suggested is good, but I urge you to not drive while under the influence of it. In higher dosages it is capable of quite strong sedation and even inducing hallucinations.

You should take a look at the water lilies Nymphaea caerulea, Nymphaea ampla or Nelumbo nucifera. They are all very mild in effect, but definitely noticeable if you are somewhat in contact with your body. You should be able to drive safely while under the influence of one of these, but seriously I can't recommend it. Please stay sober while on the road.