The ideal shape to minimize drag is a teardrop having a smooth airfoil-like shape, but it is not practical for road vehicles because of size constraints. However, researchers including Kamm found that abruptly cutting off the tail resulted in a minimal increase in drag. The reason for this is that a turbulent wake region forms behind the vertical surface at the rear of the car. This wake region mimics the effect of the tapered tail in that air in the free stream does not enter this region (avoiding boundary layer separation), therefore smooth airflow is maintained which minimizes drag.