I don't know if you are familiar with Jorton.....he's a bit of a tease.....just ignore him, he has already called this site a "pile of pants" today.
In answer to your question, I don't know, but I have wondered the same thing. I don't know if you have noticed, but it is always warmer in towns where there are a lot of vehicles.....just look at the weather reports to see that London is warmer than other places. I can't help but think that cars, buses, motorbikes etc play their part in causing such heat. I also think that if you are stuck in rush hour then you are more likely to feel the heat with the sun beating through the windshield and the lack of air moving in the car.....it's no wonder that temperatures start rising when stuck in traffic....there is nothing more infuriating.