Is a congested highway a microcosm of Global Warming?

I drive home during peak hours in the afternoon. On a sunny summer days, not long after I get off the highway and onto the back roads to home, my car's thermometer drops like 5 degrees (F) and it feels much cooler (I drive with my windows down). As a man of science I am led to believe that the increased heat on the highway is due to the above-normal amount of stagnant greenhouse ges which make the highways act like a microcosm of global warming. Is this the case or is it something much simpler, like the air around me is just the warmer exhaust from cars?