The Henry Ford includes a selection of presidential automobiles.  These all seem to have been Lincolns, unlike the current vehicle which is a Cadillac.  While the style has evolved over time, there are common themes.  The need to be seen and the requirements of the Secret Service to ride along show on the cars.  The most recent Lincoln was the car replaced by the current vehicle.  It has a folding rear bumper that agents can stand on and a pop up handrail for the them to hold on to.  The older vehicles have less of a security element.  The assassination of Kennedy changed all of that of course.