Apparently there is a Secret Service basic rule that Presidents cannot and do not drive. The only thing I've ever seen Presidents drive are a golf cart, a horse (Reagan), and that's about it. Don't know for sure, but it may have to do with the protected subject being able to be covered or pulled to safety without the distractions and time-wasting steps of stopping a car, getting it out of gear, unbuckling seat-belts.... all the things we normal drivers take for granted.