Traffic Case of the Speeding Spit

I presided over a traffic court trial of an 18-year-old charged with speeding in a residential area. The officer was a motor cop doing radar enforcement.

Radar and Lidar Traffic Enforcement Training

First, some background.  In California, in order for police officers to use radar (radio waves) or lidar (laser waves), they must complete a specialized training course with the California High Patrol (CHP). 

 Part of the training covers the maintenance, calibration, and use of the scientific instruments.

Visually Estimating the Speed of Vehicles

Another part of the training is visually estimating the speed of vehicles. Most people think that when the officer says, “I visually estimated the speed of the car at …” that the officer is just guessing. Not so. They must actually pass a test on visually estimating speeds. If they don’t pass the test, they don’t qualify to use radar or lidar.

There are officers in my jurisdiction who can visually estimate the speed of vehicles within 1-2 miles per hour. In fact, motor officers sometimes have friendly competitions to prove who can best estimate speeds. 

The Posted (Prima Facie) Safe Speed

For officers to lawfully use radar or lidar there must also be a recent speed survey of the roadway. Calculating the safe posted speed limit is a matter of scientific traffic engineering. The factors that go into that decision include: the 80th percentile of speed based on a traffic survey, the number of lanes, the curvature of the road, the roughness of the pavement, the distance between lights, the number of side streets or drive ways, the pedestrian traffic, etc.  

Speed limit signs are not based on hunches or guesses.  They are based on scientific and engineering analysis. Thus, the posted speed is the prima facie, or presumptive, safe speed

I have had traffic offenders testify that the “real” safe speed should be 10-15 miles faster than the posted speed limit. When I ask them what scientific and engineering evidence they base their opinion on, they don’t respond.

The Speeding Spit

In the present case, the motor officer testified, “I was working speed enforcement in a residential zone where the posted and safe speed is 25 mph.  I visually estimated the speed of the defendant’s car at 50 mph. I clocked his speed on the radar gun and it came back exactly at 50 mph.” (Some officers actually show the speeder the radar/lidar results. Some officer even document the results by taking a picture on their cell phone.)

The young man testified: “Your honor, there’s been a terrible mistake.  I wasn’t going 50 mph. I was only going 25 mph. But just as the officer pointed the radar gun at me, I spit out the window.”

So, the speed of the car was 25 mph, and the speed of the spit was 25 mph. When you add those together you get 50 mph.  So, the officer clocked the speed of the spit, not the speed of the car.”

The defendant was sincere and serious. His math was fine; 25 plus 25 does equal 50.  But his knowledge of physics and his common sense were lacking. “I little bit of knowledge is a dangerous thing.”

He forgot at least two things.

Trajectory/Angle of the Spit

The first thing he forgot was the trajectory or angle of the spit.

A driver doesn’t spit forward. There’s a windshield there. Drivers spit sideways, out the window. Drivers spit diagonally or perpendicular to the moving car. The angle of the spit would significantly reduce the forward motion, to close to zero. 

Wind Resistance

The second thing the young speeder forgot was the principle of wind resistance.  

You don’t spit directly into the wind, or it will hit you in the face. As soon as you spit out the window of the moving car, the spit looks like is goes backwards. Because of wind resistance, the spit goes slower than the car, not faster.  


I had to keep my hand over my mouth to keep from out laughing out loud while the teenage driver was testifying.  (The officer was chuckling quietly.)  I calmed myself and ruled, “That is very interesting, but I am going to have to find you guilty.”

I decided not to force him to listen to a boring lecture by me on the principles of trajectory and wind resistance. He had already been punished enough.

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