Threshold braking is one of those concepts that has been misunderstood for decades in the police and protective driving community. It has often been confused with another method of braking used in racing – trail braking.
In my racing days, we used trail braking as a method to maximize the vehicle’s speed through a corner.
Trail braking works great on a racetrack – in a non-Antilock Braking System (ABS) vehicle.
Threshold braking is a method drivers use to stop a street vehicle in the least amount of time and distance. There is a substantial difference between the two braking methods. Trail braking works, but threshold braking does not.
We have heard that there are driver training providers teaching students to use Threshold Braking in a vehicle equipped with ABS. Not a good idea – that is a lawsuit looking for a place to happen.
For those who feel that saying that threshold braking does not work is blasphemy, we ask you to please keep on reading – I think we will change your mind.
Measurement is Key
Any method used for driver training, including braking, must be measured to ensure it works. When we first started instructing police departments in the early 70s, we quickly learned that you could not conduct braking exercises without accurately measuring speed – stopping distance, and the time needed to stop the vehicle. And the method of braking has to work in an emergency scenario.
An emergency scenario is a minimum amount of time and distance to stop. If the driver has a lot of time and distance to stop, it’s not an emergency. Fifty years of conducting training have told me that one of the most critical skills needs to be taught and measured.
And now, with the advent of ABS, threshold braking is useless.
We could go into the “Why it doesn’t work” explanation, but a while back, we shared a YouTube that explains why it doesn’t work much better than we can. This video addresses a common misconception within the car community and trainers – that threshold braking is better than ABS braking.
The video is from the Engineering Explained YouTube Channel. The narrator is Jason Fenske, a graduated mechanical engineer from North Carolina State University, and he makes videos on automotive engineering topics.
Threshold braking is a driving skill that needs some explanation.
Before we go deeper into Threshold braking and ABS, let us explain just what threshold braking is.
The friction created between the tire and the road makes a car stop. The maximum adhesion (which in turn is maximum braking) available from a tire (therefore a car) is when the tire is at 80 percent of the slip or the threshold, hence the phrase “Threshold Braking.”
After applying the brakes, slip is the amount of the tire that slows down.
This is a measurement of how much the tire has stopped rolling. If the wheel is not rolling and sliding across the road surface, it is 100 percent locked and has no slip. The coefficient of friction (adhesion) is higher at 80 percent of slip than it is if the tire is locked (not rolling).
Why Threshold Braking Does Not Work
The problem is how quickly you can find that 80 percent of the slip point (the threshold), and can the driver find it the first time they hit the pedal? If you are traveling 40 mph – 64.4 KPH, you are traveling 60 feet or 18.3 Meters per second. If it takes a driver one second to find the threshold, they have traveled 60 feet or 18.3 Meters looking for it.
The question is, How much less friction do you have between the threshold and lock? The answer is a difference of .15 or 17 percent. So we can stop a car 17 percent quicker with threshold braking than if the wheels are locked up.
ABS and Threshold Braking in a real-world scenario.
These excerpts are from our analysis of the Omar García Harfuch (OGH) ambush.
A video and photos showed that the ambushed vehicle initially stopped 20 Feet/ 6Meters in front of the blocking vehicle.
Our research required us to determine the rate of deceleration and the amount of time and distance the driver could have had to stop an armored B6 Suburban.
Using our Decision Sight Distance analysis, the driver had 120 feet/ 36.6 Meters to stop the armored vehicle before arriving at the large truck blocking the intersection. The driver stopped 20 feet/6.1 Meters in front of the truck. A quick calculation tells us that the driver stopped the vehicle at 100 feet or 30.5 Meters.
When we replicated the incident using the braking numbers from our analysis, the VDI testing team found interesting data.
A VDI test driver with years of experience conducting driver training and teaching students threshold braking with an ABS-equipped vehicle drove into the replicated OGH ambush with an armored vehicle.
Using the threshold method, the driver took 160 Feet or 48 Meters to stop the vehicle.
In the exact vehicle, the same scenario, the same driver braking hard with no threshold used 94 Feet or 28 Meters to stop the vehicle, representing a difference of 70% less distance to stop the vehicle.
If the driver of OGH’s vehicle had tried to use threshold braking, the event’s outcome would have been significantly different. Our testing shows or indicates that the suburban would have been under the truck if the driver tried to threshold brake with ABS would’ve been a completely different result in the event.
Lessons
- Accurately measure vehicle braking.
- Understand the effects of the ADAS systems, and
- The limitations it may provide for the security vs. safety function
- Discuss the braking methodology first; all vehicles used for training must have ABS.
In confirming our Forensic Analysis of the OGH ambush, the VDI testing team found interesting data concerning the braking method.
Braking Distance – Lesson Points and Recommendations
As a trainer, do not teach threshold breaking with an ABS vehicle. We cannot imagine an executive vehicle that does not have ABS brakes.
As a potential student or purchaser of protective driving training, we suggest you ensure that you or your employee are not attending a program that instructs a student to threshold brake with an ABS vehicle.
Measuring the entrance speed accurately into all braking exercises is a must, as well as a computer or radar gun. For example, with the experienced driver increasing the speed by 10%, which would bring it up to 44 MPH or 71KPH, the stopping distance would increase by 20% to 113 Feet or 34.5 Meters which would have produced an entirely different result.
This is an issue that can create liability problems. The training provider is training you or your employees to NOT USE an ADAS system proven to prevent accidents and save lives.
Training needs to change to adapt to vehicle technology – On almost all executive vehicles, some computers and algorithms will limit the driver’s ability to stop the car and turn the car – driver training needs to reflect the increased use of computers and executive vehicles.
If you’re driving in an armored vehicle, determine the vehicle’s stopping distance by testing the vehicle.
The time to know that you need 90 Feet/27.4 Meters to stop your armored vehicle is before a truck blocks the intersection 80 Feet/24.4 Meters in front of you.