
The picture above shows the air pressure on the smart as it travels forwards.
This thin layer of compressed high pressure air diverts a lot of the dirt and
bugs over the car keeping it averagely clean.

As the smart moves forwards it pushes the air around the body. An area of low pressure is created behind the car. Low pressure will pull air from the surroundings to fill the gap drastically altering the air flow around the rear.

The change in direction of the air creates 'eddies' which pull the dirt off
the road, soot from the exhaust and passing bugs into the rear of the car.
This can be worsened if you have a non standard exhaust as the emissions get caught in
the low pressure eddies as apposed to being dispersed by the airflow from under the car.