Snapping On The Road
I suggest you do, because you never can tell when a picture will roll by. It might be a woman wrapped up in snarling dogs or a cyclist riding wrong-way through 4 lanes of the Freeway or, as you can see, replacement wheels for shopping trolleys being conveyed along Leach Highway early in the morning.
Do not endanger your life by trying to set up a tripod and a view camera in he front seat of the car as you go along. It is useless to spot-meter the scene and calculate the Zone-System exposure between lights - even slow moving trucks go faster than Ansel Adams ever did.
Get yourself a Fuji X-10 or X-20, a Canon G15 or G15, or one of the fine Nikon or Olympus compact cameras. You can eschew the viewfinder because you should keep looking forward as you are driving but set the thing to about 400 ISO, AF-C and programmed exposure and then just blaze away. Today's cameras will do far more than ever before and you could be rewarded with some REAL street photography.
BTW: If you are a passenger keep your arms and legs inside the car as you snap. No good winning a photography award if you don't have the hand to receive it...