Karman vortex

Any sufficiently blunt object placed in a steadily flowing fluid will disrupt the flow of the fluid in a peculiar manner. When the boundary layer of the fluid/body interface becomes separated from the body, a periodic train of vortices tends to form. This is especially true for simple body geometries. The rate of vortex generation is proportional to the ratio of the flowspeed to the width of the body, or f_K=U/D. These periodic vortices are Karman vortices, and there are two easy ways to see them at work.

The first method is to drag a spoon through a cup of recently-creamed coffee. The cream will form patterns in the coffee as it swirls into the wake of the spoon.

Second, follow an 18-wheel truck on the highway. Get close enough behind it, and your radio antenna will start to sway back and forth. As you get closer to the body, the antenna will stop oscillating as you enter the truck's wake. Slow down a bit, and get back to the point where the antenna is swinging back and forth at a fixed frequency. This frequency is the Karman frequency, and is caused by periodic vortices alternatingly spinning off the left and right rear edges of the truck. Given the truck's width, and the frequency of the antenna sway, you can compute your current speed.

Of course, I doubt that the judge will accept that as an excuse in court, after you've been pulled over for speeding and tailgating...