Physicist uses math to avoid traffic penalty

In the late 80s, VASCAR was heavily used on I-95 between Baltimore and DC - a route I used to travel a lot. Coming from Baltimore to DC, I was pulled over and given a citation for 67 in a 55 or something similar.

VASCAR is a method that is used with a stopwatch and a simple computer that divides distance travelled by the elapsed time. To avoid too much error, markings on the pavement are usually fixed and marked at a tenth of a mile or a quarter of a mile interval. In this case, they were using the time at which the car left the shadow of a bridge, to two traffic cones on the side of the road on an uphill stretch of I-95 - http://g.co/maps/x6az8

When I got the ticket, I asked two things - how did you know how fast I was going and when did you set up? He answered that they had started writing tickets around 9:45 that morning and the ticket was issued at 2:15 in the afternoon. Instantly, I formed my alibi.

Since we knew that the lines on the street weren't painted, we knew that the sun travelling across the sky would alter the point at which the car came out of the shadow, and, we were using two traffic cones on the side of the road that didn't cast much of a shadow to judge the position of the car. I forget the math based on the time that they measured it, but, it was something on the order of 12-18mph error based on the angle of the sun, time of the year and their setup time versus when I was stopped. I had pictures, a handout showing the math used referencing geometry books with footnotes explaining the sun's positional changes, the distance that the shadow would have moved over that time period, the increasing error throughout the day.

The officer stated that his VASCAR computer had been calibrated and he was a certified operator and all of my math was just mumbo-jumbo. They took a five minute recess - unheard of in traffic court - came back, and issued a not guilty on my case. The judge then asked if anyone else there had received tickets on that date from any of the six or seven officers that had issued tickets, and about 25 people raised their hands - all having their tickets dropped.

In point of fact, he probably was within 3mph of my actual speed as all of my measurements had been done based on a 1/10th of a mile rather than the quarter mile that they had used that day, but, there were a lot of happy people that day. It is a good thing I didn't realize that the cones were set at 1/4 mile, but, there still would have been some error.