The older Cessna's (like 172,152) have an average wing span of about 10m, which would need about 2 lanes of some highway or some other road and would be easily available on some National Highway. The taxiing speed that Cessna can achieve is pretty decent. So, what problems would a person face trying to use a Cessna as a Car, one of them could be space, but what are some other problems, barring any regulatory/legal issues? Mainly I am demanding an answer from fuel, speed, and safety point of view.
Even assuming there is room - wings and the propellor could quite easily hit something or a pedestrian not looking where they were going... etc etc. An aeroplane is quite difficult to steer on the ground because the flight controls don't work very well if at all at low speeds - you use the steerable nosewheel which is connected to the rudder pedals and also the brakes (one on each rudder pedal that affect the main wheels).
Due to the large areas of body and not least the wings and tail it would be affected by winds at street corners for example. It would have to keep its speed fairly low (10-20mph). Any faster and it would be very stressful for the pilot and quite easy to lose control of it. It would also be difficult to judge clearances from other objects at higher speeds. Cars don't have wings stuck out 15 feet either side of the driver!
Having said that I am sure it is far easier to drive an aeroplane on the ground than it is a car in the air!