A previous article discussed the basics of driving at night - this article will cover a specific problem associated with driving in darkness - over driving you're headlights.
To illustrate the dangers of overdriving your headlights we will look at scenarios, with the objective of creating information that can be used as teaching points. To accomplish this we need three numbers, vehicle stopping distance, headlight illumination distance, and reaction time.
Reaction Time
In a previous article titled "Reaction Time" we discussed reaction times when the event is a surprise. And surprise would describe a driver moving faster than they can see and having an emergency pop into their vision. Research has shown, and in our scenarios we will use, that the best estimate in a surprise scenario is a reaction time of 1.5 seconds.
Braking Distances
Using the numbers from the 2008 Michigan State Police test, the average stopping distance for a Police package Crown Vic is 143 feet at 60 MPH. Using some math I computed the estimated stopping distance for the Crown Vic at speeds other than 60 MPH. At
- 40 MPH Stopping Distance would be 64 Feet
- 50 MPH Stopping Distance would be 99 Feet
- 70 MPH Stopping Distance would be 194 Feet
Illumination Distance
We will use what is considered to be the average illumination distances for low and high beam lights, which are 350 feet for high beams and 180 feet for low beams.
Knowing the stopping distance of the Crown Vic, the illumination distances the headlights supply, and driver reaction time we can produce scenarios that will make good teaching points.
SCENARIO ONE - LOW BEAMS
With low beams a driver has 180 feet of vision to work with. If the vehicles speed is 40 MPH it is moving at the rate of 58.8 Feet/Sec. The driver is confronted with an emergency at the edge of their 180 foot vision mark. If the driver's reaction time is 1.5 seconds the driver would use up 88 feet (58.8 x 1.5) of the 180 feet getting to the brake pedal, leaving the driver with 92 feet (180 - 88) to stop a vehicle that needs 64 feet to stop, leaving the driver with 28 feet (92 - 64) to spare. Sounds like a lot of room? Consider that initially the driver was moving at the rate of 58.8 Ft/Sec it's not much of a cushion.
Same scenario except the driver's reaction time is increased by a half a second? Reaction time is now 2 seconds, which, at 40 MPH, translates into 117.6 feet to reach the brake pedal. (58.8 x 2) Which will leave the driver with about 62 feet (180 - 117.6) to stop a vehicle that needs 64 feet to stop, the driver will be 2 feet (62 - 64) into the emergency - not good.
It is a safe bet to say that the threshold speed on a poorly lit road with low beams, under the best conditions, is 40 MPH.
SCENARIO TWO - HIGH BEAMS
The same exercises with high beams - the driver has an average of 350 feet to work with. Move the speed up to 60 MPH or 88.2 Feet/Sec. The driver is confronted with an emergency at the edge of their 350 foot vision mark. The drivers 1.5 seconds of reaction time will use up 132 feet to get to the brake pedal (88.2 X 1.5). The driver has 218 feet (350 - 132) to stop a vehicle that needs 143 feet to stop, leaving the driver with 75 feet (218 - 143) to spare. Sounds good - but consider that the initial speed is 88 Feet/Sec - again not much of a cushion.
Raise the driver reaction time a second, reaction time is now 2.5 seconds, which, at 60 MPH, translates into 221 feet to reach the brake pedal (88.2 x 2.5). Which will leave the driver with about 129 feet (350 - 221) to stop a vehicle that needs 143 feet to stop, which would leave the driver 14 feet (129 - 143) into the emergency - not good.
Move to 70 MPH or 103 Ft/Sec, at 1.5 seconds reaction time it will take 154 feet (103 x 1.5) to reach the brake pedal which leaves 196 feet (350 - 154) to stop a vehicle that takes 194 feet to stop - again not good.
On a poorly lit road with everything in the drivers favor 60 MPH is about it for driving with high beams, at 65 your tempting fate at 70 it can turn into a disaster.
There are many variables that would change the outcome of the scenarios mentioned above, and none of them will make things better - dirty or misaligned headlights - changes in reaction time - braking capability of the vehicle. In a pursuit where the brakes are being used often, you can be assured that the braking capability of the vehicle is decreasing.
A suggestion for those running EVOC programs - measure the headlight distance - high and low beams, of not only the training vehicle but also a sample of the patrol vehicles. And if you haven't done it already measure the braking distances of the training and a sample of the patrol vehicles. Work out the above scenarios with the numbers for your vehicles. It would be information you can pass on to your students.
CORRECTION
In the last article, Driving in Bad Weather I messed up - what I meant to say and what came out in the article were not the same. The point I wanted to make was that stopping distance in any condition is determined by the CF created by the tire road combination. I don't know why I didn't just say that. I attempted to be graphic, talking about pitcher’s mound, football fields and 4WD, screwed it up, and all I did was create the wrong impression, I apologize for doing that.
The point is that, once the brakes are engaged it's all about tires on the road surface.
