Have you been on Ambergris Caye, its a little different.
Car ownership probably doesn't have much consideration over gas prices, very short distances between points on the island that have roads.
I'm not sure if we have anything that would qualify as public transportation except the Coastal Express Water Taxi and its usually taking people places a car couldn't get to.
Gas prices on the island seem to effect Boat operators the most. Boats are used to entertain the tourist so it raises the prices tourist pay for things like Fishing, Snorkeling , Diving, Mayan Ruin trips, Birdwatching Trips, etc.