My flatmate told me during the week that because many Americans are so grossly overweight now that their cars have to burn double the fuel to get them from A to B.
If you extrapolate that, isnt it then just as environmentally unfriendly to have that extra doghnut or slice of pizza as it is to board a long-haul flight?
Though I have not read the article myself nor heard the news story, as it was relayed to me - Americans are also buying larger vehicles (SUVs, minivans, etc.) to accommodate their larger size. These vehicles are not as fuel efficient as a smaller car. Add their own weight to the weight of the car and it's already decreased miles/gallon fuel consumption - you end up with a vehicle that can be as gas-guzzling as a large V-8 from the 60s.
As for the taxi comments... I remember a time when taxis were a shared means of transportation. That is, more than one person would be picked up at a location and the amount would be charged for the distance each passenger would travel. That is not the case anymore. Now, it tends to be one passenger (unless there are more in your party) and many more cabs needed to accommodate.
I do agree with Peter that if more people used the taxi services, less actual cars would be consuming fuel. But again, those who use taxis (or any type of livery service) are generally selfish and would be hard pressed to consider sharing a ride with someone else.