by Paul Raveling
Originally published in the Waterford HOA newsletter, summer 2000
 

Four traffic accidents within a mile of Waterford have produced four fatalities in April and May.  Three of these were on roads that Waterford residents use for access to our community, one was a bicycle accident in Marina Village.  Speed was a factor in all of these fatalities, alcohol contributed to two.

This article examines one of these fatalities, the April 2 death of an 18-year-old at the intersection of Lakehills Drive and Salmon Falls Road.  It relates this case to basic physics to point out that maneuverability decreases dramatically as speed increases.

With only aftermath as evidence we cannot be sure that the driver who died is the one who did a burnout on a lawn before exiting Waterford at Cromwell Court around midnight, but it seems likely. He accelerated to a high speed while southbound on Lakehills Drive, probably in the range of 80 to 100 m.p.h., perhaps even more.

As he approached the last crest, where Lakehills falls off downhill toward Salmon Falls Road, he moved to the left side of the road and initiated heavy braking, to anticipate a high speed turn onto Salmon Falls.  As he initiated the turn, his sport-utility's weight transferred from the right side to the left side, producing skid marks from only the right-side tires.

His speed was still so excessive that his turn radius was enormous. The partial skid produced a heavy mark from the right front tire, a much lighter mark from the right rear, and no marks at all from the left side tires.  This suggests that he was already braking heavily before the turn, using as much braking as he could without breaking the tires loose.

The extreme turn radius suggests of that his speed was still extremely high when he attempted to turn, probably at least 60 to 80 m.p.h., possibly even faster.  A change in the skid marks at the far edge of the pavement is consistent with this, possibly indicating a point where his left front suspension bottomed out.

At this point he left the road, crashed through three small trees and fencing around the home at that site, impacted the far side of a gulley, and finally came to rest at the bottom.  The driver was impaled by a timber from the fence.

This accident appears consistent with a pattern common to speed-related accidents, few drivers have the judgement skills needed at high speeds.   It's not natural to humans to anticipate the exponential effects of speed, which are minor at low speeds but grow enormously at higher speeds.

First, minimum stopping distance is proportional to the square of speed, not just to speed itself.  If you need 25 feet to stop from 25 m.p.h., then you need 400 feet to stop from 100 m.p.h.: Multiplying speed by 4 multiplies stopping distance by 16.  In this case the driver who died probably misjudged how far in advance he'd need to begin braking, causing him to carry far too much speed into the turn.  The first accompanying graph illustrates distance travelled as a function of speed with separate traces for several initial speeds. This assumes braking at .82 G, a value measured for a light truck that would probably be typical for maximum braking in a sport utility such as the one in this accident.

Third, kinetic energy also is proportional to the square of speed. Kinetic energy in a collision becomes impact energy that WILL be dissipated in one way or another.  If you imagine driving into an obstacle at 25 m.p.h., then impact at 100 isn't 4 times more destructive -- it's 16 times more destructive.  The graph of kinetic energy as a function of speed has the same parabolic shape as the curves above for turn radiuse, only the labeling of axes will differ.

Another judgement problem doesn't require physics to understand. Suppose this accident scenario had happened at a different time of day; someone returning home could have turned onto Lakehills Drive, accelerating up the first hill only to find a sport-utility approaching head-on in the same lane at a closing speed of nearly 150 m.p.h.  Since the fast vehicle has virtually no ability to maneuver, how many of us are ready to take evasive action fast enough to avoid a collision?  "Taking the ditch" on Lakehills is almost certain to mean damage to our own car, sport utility, or truck; will we be sufficiently prepared to limit our own damage to non-lethal levels?

Suppose the fast driver had been slower but was still so fast that he had no option but to run the stop sign at Salmon Falls and finish his high speed turn on the wrong side of the road. Again, how many of us will be ready to get out of the way, even if it means that we have to be the ones who take the ditch?

Alternatively, how can we make aggressive drivers understand how easy it is to die?