Another real-world solar vehicle:
http://www.solarvan.co.uk/
16 square feet is only 1.5 metres squared, easily achieved on the roof only (without bothering with the hood etc).
SunPower A300 cells are 20.4% efficient, and the US year-round average incident energy is 5.5 kWh per m2 per day. Thus, in a day you could collect 1.5 x 5.5 x 0.204 = 1.7 kWh.
1.7 kWh doesn't sound like much, but it equates to 7 miles per day. Too much for the existing battery (which isn't up to this type of use), so solar only works with PHEVs with bigger batteries.
Ultimately roof mounted cells are not the ideal, the best solar option is to use automatic extending "car-covers", made from flexible
nano-solar type material. Keeps the car cool and up to 7 metres squared (30 miles per day EV).