In Part One of this short series, we explored the rudiments of reaction time and braking distance. The arithmetic for understanding both concepts was learned in the third grade (multiplication), fourth grade (long division), seventh grade (fractions) and eleventh grade (drivers' education).
Most bus and motorcoach drivers have high school educations, during which time they presumably learned the four processes noted above. But they often do not retain these processes, and cannot convert them into safe driving practices. And their training rarely acknowledges the existence of these disciplines, much less their importance, much less the need to integrate them into the training.
Sloppily understanding and applying these principles may "get by" on a gray, cloudy day. But once the sun goes down, even when the moon is full and the sky cloudless, this arithmetic must go a step further. This is because the vehicles' low beams illuminate only a limited distance in front of it, and the faster the vehicle is traveling, the faster its movement gobbles up this space.
Outdriving One's Headlights
Understanding the simple arithmetic noted is a starting point for the more-difficult calculation of nighttime visibility. To see both further ahead and more clearly, drivers know they can engage their high beams, or "brights." But the glare from these high beams can dangerously impede the vision of, oncoming drivers or motorists. So good drivers learn to drive and see with their low beams. As a consequence, the distance in front of the vehicle which the low beams illuminate is the principal limiting factor of safe nighttime travel speed.
This distance is most important as it relates to reaction time (see Part 1 of this series):
- At 60 mph, a driver's two-second reaction time will propel the bus 176 feet.
- At 70 mph, a driver's two-second reaction time will propel it 205 feet (29 feet further).
This relationship is "linear." Braking distance is crudely "geometric." Twice as fast translates crudely into four times as far. When then the bus must brake to a stop:
- At 60 mph, one must add another 216 feet to brake it to a stop.
- At 70 mph, one must add more than 300 more feet to brake it to a stop (more than 75 feet further)
For a given span of illumination, governed by a combination of speed and low-beam headlight alignment, there is a speed beyond which, when an object is observed in front of the bus, the driver cannot stop before striking it. This seems simple enough. When the distance traveled during reaction time exceeds the span of low-beam illumination, the driver has effectively outdriven his or her headlights. Braking distance is not as important since, during reaction time, an awake, alert and skilled driver may often be able to swerve and avoid the collision. But roadway constraints may not always make this possible. And while defensive driving is even more important at night, it is also more challenging, and certain parts of it (e.g., observing space surrounding the vehicle which may not be illuminated) may be difficult or impossible.
Otherwise, like most things, an incident where a driver outdrove his or her headlights is not this simple. This is because a number of factors contribute to an incident. And some of them directly affect this phenomenon.
Training, Monitoring and Enforcement
While the arithmetic on which all these relationships depends is rudimentary, it is not that simple to apply it to travel speed. It would seem reasonable and prudent for every driver to know how far in front of his or her bus its low beams illuminate. Yet realistically, few drivers have precise knowledge of this. The concept is rarely included in training. And when it is monitored (e.g., by a GPS device), appropriate speed is usually measured against speed limits - not the travel speed at which the driver can stop before colliding with an object suddenly illuminated by the vehicle's low beams.
Nor do regulatory requirements consider this relationship. Dumbed down to speed limits, regulations only measure speed against an abstraction which does not factor in many things, such as visibility, ambient illumination, headlamp illumination, road surface, moonlight, clouds, fog or many similar factors that genuinely affect the safety of traveling a given speed for conditions. When something untoward occurs, such considerations enter the world of reconstruction. But these same factors overwhelm a police officer. In that realm, all these factors get slopped together into a regulatory blob known as driving "too fast for conditions."
Tilt and Scope
During every vehicle's annual inspection, the tilt and scope of both high beams and low beams are adjusted. The distance in front of a vehicle which the low beams illuminates varies slightly from state to state. It varies wildly if one tries to "google it." The language about high beams is elusive and fuzzy (see http://clearlyvisiblepresentations.homestead.com/Low_Beam_vs_High_Beam_by_State.pdf that article). Descriptions of low beam requirements are obtuse.
Regardless, even in a reasonable and prudent world, drivers may not know the distance in front of their vehicles which their low beams illuminate, much less can calculate how fast they can travel to stop in time to avoid striking things their low beams suddenly illuminate.
Bright and Brighter
Just as state inspectors and many mechanics to, a bright driver knows his or her state's requirements for low beam illumination (i.e., to align the headlights correctly). A brighter driver measures the actual distance of this illumination: Drive to some dark, deserted parking lot and simply measure this distance. If the distance is much below or above the state-required distance, have the headlamps realigned to extend their field of illumination. If the distance is above the required distance, you can travel a bit faster, but only at the expense of shining your headlights too brightly into the vision of an oncoming driver or motorist.
Brighter still, most drivers know of practices which can sometimes decrease the distance covered during reaction time or braking. One practice is "covering the brakes," which eliminates roughly ¾ of a second of reaction time (and the distance the vehicle travels during this time). And any vehicle can brake more sharply on a given surface when traveling uphill. But these options are not always available. The brightest drivers remember the math, and apply it. In simple terms, they know how far their low beams illuminate the roadway in front of them, and they know how fast they can travel without "outdriving their headlights."
Tough Challenges and Tough Justice
A driver can be wide awake, alert, healthy, well-rested, sober, drug-free, untroubled mentally and emotionally, and deeply tuned into safe, responsible driving. But if that driver outdrives his or her headlights by only a few feet, he or she can turn out the lights of a pedestrian poised just a few feet beyond the driver's field of vision.
Given the spectrum of variables which affect safe travel speed, it may seem harsh to blame a driver for traveling a few miles per hour too fast. And certainly a range of other variables (better tire tread and inflation, better roadway surface, larger and better-maintained brakes, better signage, bio-sensitive driver assignment or even more moonlight) may render this speed considerably less risky. Similarly, other tweaks can often help a driver avert a collision.
In a courtroom, sometimes these factors can be argued. And clearly things which a driver did well may count. But as hard as the math may be, and as numerous as the other factors are which could have been tweaked, the driver is generally responsible, or at least largely responsible, for doing the harm rather than doing the math.
It is better to do the math. And it is always better to do what one can control.