Why Old Driver’s Ed Instructors Used To Fire Gun Barrels Loaded With Shells To Teach New Drivers

If you’ve been through driver’s ed or other mandatory testing to get your license, you’ve probably learned a bit about reaction times and braking distances. It’s important knowledge, after all, for new drivers to understand how speed and human reactions impact their ability to stop in a hurry. Today, we learn these lessons from books. However, once upon a time, a simple firearm device was used to teach these lessons instead.

The device is known by a variety of names, but the American Automobile Association (AAA) calls it the Brake Reaction Detonator. It was first developed in 1937 by one Mr. Earl Leroy Allgaier. Allgaier was a graduate of Miller High School (Go Rustlers!), and joined AAA in in 1936 after achieving a Bachelor’s degree in Civil Engineering and Masters Degrees in Highway Engineering and Education and Psychology. Over a 37-year career with AAA, Allgaier made it his life’s work to develop tools and techniques for driver training and education. The brake reaction detonator was one of his greatest successes in this area.

The concept is simple. Mount a pair of gun barrels on the bumper of a car. Pick the front or rear bumper, it doesn’t matter, as long as both barrels are mounted at the same point. The barrels are then loaded with cartridges that shoot out a burst of chalk when fired. The driver of the vehicle is then told to drive at a steady speed. At a random point, an instructor triggers one of the barrels to fire. This leaves a chalk mark on the ground, and the loud crack of the gun is a signal for the driver to brake. When the driver actually hits the pedal, this triggers the second barrel to fire its chalk, leaving a second mark on the ground.

Measuring between the two marks indicates the distance covered while the driver reacted. By dividing that distance by the known constant speed of the vehicle prior to braking, it’s possible to determine the driver’s reaction time. Measuring from the second mark to the car’s final position gets the actual braking distance, while measuring from the first mark to the car’s position gets the total stopping distance including human reaction time.

The device received US Patent # 2,177,312 in 1939. AAA widely touted the benefits of the device, advertising it in fliers. You can buy just such a flier on eBay for the low cost of $6.37 plus shipping. I’m kind of tempted to purchase it myself and frame it, but they want $22.36 to ship it to Australia. In any case, you can view it below.

The benefits of this testing device are that it aids in accurate and precise measurement of braking distances and reaction times. Traditionally, such measurements involved using lines marked on pavement or cones, and instructing a driver to hit the brakes upon passing the marker. These methods generally introduced a great deal of human variation which spoiled any data collected. By automating the marking of the brake point by firing a chalk mark at the ground, this problem was eliminated.

Indeed, other researchers greatly appreciated the value of this invention. In 1954, H.J.H. Starks and R.D. Lister published a paper entitled Braking Performance of Motor Vehicles, in which they discussed a variety of practical braking experiments. They noted the difficulty in making accurate measurements at the time, particularly at the moment braking begins.

It is not perhaps fully appreciated that the method used to indicate the position of the vehicle at the instant of braking must be very rapid in action. A delay of 1/5th of a second in recording the position of the vehicle means that the recorded braking distance is 9 feet short when braking from 30 m.p.h.

The use of a brake reaction detonator solved this problem. With the barrels mounted a few inches above the ground, the delay between hitting the brake pedal and the chalk hitting the ground was only “a few milliseconds”, which was negligible for the researcher’s purposes. By contrast, a fifth of a second is 0.2 seconds, or 200 milliseconds.

Of course, for measuring reaction time between the two chalk marks, the delay would be essentially equal. Each chalk blast would take the same time to travel the same distance to the ground. The delay would only matter in the case of measuring to the car’s position. But let’s do the math. At 60 miles an hour, you’re traveling 1 mile a minute, or 88 feet per second. That’s 0.088 feet per millisecond, or 1 inch. So, even in high-speed braking tests, your measurements would probably only be off by a few inches at most due to the time taken for the chalk blast to hit the ground.

This simple piece of technology came to our attention when spotted by our own Laurence Rogers, who happened to see this post from the New South Wales Historic Police Museum. The tool, seen mounted to a VJ Chrysler Valiant, was used to demonstrate the effect of speed on braking distance. As any physicist will tell you, the kinetic energy of a moving vehicle increases with the square of speed. Thus, if you double your speed, you increase the vehicle’s kinetic energy by four times. Wiping off that kinetic energy in braking takes four times as long, so the braking distance is four times longer.

However, it bears noting that total stopping distance, including reaction time, usually isn’t quite as much longer. That’s because the distance traveled during the driver’s reaction time is only twice as long at double the speed. For example, in figures from the UK Highway Code, the braking distance of a vehicle is approximately 20 feet at 20 mph. It’s approximately four times greater, or around 80 feet, at 40 mph. Bring reaction time into the equation, though, and the relationship differs. Total stopping distance is quoted as 40 feet at 20 mph, or around 118 feet at 40 mph—roughly three times as much.

Such a device also starred in this amazing Australian film on drunk driving from 1978. Produced for the Office of Road Safety, it discusses how alcohol affects the body and the impact this has on driving. In a wild move, they had a pair of regular Australian youths drink four beers in an hour (!!!) to reach the then-legal limit—a blood alcohol concentration of 0.08 g%. The limit was later reduced to 0.05 g% in 1991. They were then put behind the wheel.

The device was mounted to a CM Chrysler Valiant for testing. The drivers were instructed to brake as quickly as possible upon hearing the first chalk mark being fired. Keeping the car at 60 km/h (37 mph) on the straights, Gary was able to respond within 14.7 meters, or 48.2 feet. Assuming he was bang on 60 km/h, his reaction time was 0.88 seconds.

His mate Fred did far worse. After four cans of beer, Fred entirely missed his cue to brake the first time, laughing at his mistake. Amazingly, the test crew were game to get back in the car with Fred, who was clearly intoxicated at this point. On a second attempt, he reacted after 27 meters, or 88.6 feet. Again, assuming he was bang on 60 km/h (perhaps unlikely), he reacted by hitting the brakes after 1.6 seconds.

Of course, I couldn’t take those numbers at face value and move on with my workday. Instead, I spent an hour downloading the video and timed the gap between the gunshots. Assuming no frame rate irregularities, these times should be valid, and can give us idea of the speeds the two drivers were travelling at.

The video runs at 25 frames per second, giving us a measurement resolution of 0.04 seconds. In my analysis, Gary took 20 frames to react, or 0.8 seconds. With a distance between the marks of 14.7 meters, he might have been going a tad fast at 66.15 km/h (41 mph). Fred took 40 frames to react, or 1.6 seconds. He might have been feeling the effects of alcohol, and very slow to respond. However, the car traveled 27 meters in those 1.6 seconds, putting his speed at bang on at 60.75 km/h!

Both drivers did far better when sober. Gary took only 6.7 meters (22 feet) to react when sober, while Fred achieved 9 meters (29.5 feet).

The video of the sober attempts may not be quite accurate here though. Gary appears to react in 8 frames, or 0.32 seconds, which would suggest he was traveling at 20.93 meters per second given the 6.7 meters between the marks. That’s 75 km/h, or 46 mph. Meanwhile, Fred appears to react in 0.24 seconds. Given he’s said to have reacted within 9 meters, that would suggest he was traveling at 37.5 meters per second, or 135 km/h (83 mph). Either I’ve tripped over my maths here, or there’s something amiss with the video in this section, like a framerate mismatch.

In any case, I wouldn’t take a ride home from the pub from Gary or Fred. Gun to my head, though, I’d go with Gary every time.

The concept behind the brake reaction detonator still works today. Physics hasn’t changed, after all. However, most scientific testing done on automobiles is now done with more advanced electronic telemetry, using high-speed dataloggers and video and all kinds of fancy sensors. Measuring chalk marks on the road with a ruler is all just a bit old hat, now.

Regardless, this wonderful invention by Mr. Earl Leroy Allgaier taught thousands, if not millions, all about braking distances and road safety. For that reason, it ought to be celebrated as the noisy, chalky success that it was. Huzzah!

Image credits: eBay, UK Highway Code, NFSA Films via YouTube screenshot