The clouds are water droplets that are condensing as the arrive into the cold air. The "smoking" cars have not been driven very far, and the exhaust system is still cold.

Burning gasoline produces water, among other things. When the car first starts, this water condenses onto the cold exhaust system, warming it up. After the car has driven several miles, the pipes get hot enough that all of the condensed water is evaporated again. During this part of the trip, the gases coming out of the exhaust are not too hot and fully saturated with water. So condensation begins as soon as they reach the outside air.

After the exhaust system warms up, the exhaust gases come out less saturated (there being no excess water in the system to evaporate) and hotter. Mixing cold air with the exhaust cools it down, but decreases the concentration of water. The gases might not ever be above saturation, so that no condensation occurs.

The cloud does not persist. It appears at the exhaust pipe, travels a few feet, and disappears. The air is not saturated with water, and the little drops can evaporate as fast as they condensed.

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