Wednesday, October 29, 2014

2-wheeled vehicle accidents--Forensic Engineering

I am currently doing a 4-week rotation in the OCME--Office of the Chief Medical Examiner. I'm learning so many cool things. However, for my many triathlete and road biking friends, I wanted to put a little post about what I have learned about motorcycle accidents from the Forensic Engineering book I am reading.

Per-mile, motorcycles have ~2.6x more accidents than cars. However, as many more people drive cars, motorcycles only account for 1.3% of all MVAs. However, motorcycle accidents cause at least 10% of all motor vehicle deaths and motorcycles have ~ 20x more fatalities per mile driven than cars.
The greater fatalities can be explained by the fact that there is nothing but the impact of body against the ground to dissipate all of the kinetic energy from the crash (remember kinetic energy = 1/2mv^2, with m - mass & v = velocity).

 The greater number of accidents, however, is not as well explained. It can be argued that there should be fewer accidents as "fewer infirm and feeble persons drive motorcycles than cars" hence "the average motorcyclist should have better reflexes and therefore react faster in accident situations" 

One example of a common accident is when a car is making a left turn into a driveway or a cross street and hits a motorcyclist heading straight. A major factor within these accidents seems to be the perception of the motorcyclist by the driver-who doesn't see the motorcyclist or only sees the motorcyclist when it is too late.

Perception of headlights. People can use the rate of change of angle between the two headlights to estimate the distance of the car and the speed of the car. Further there is a linear correlation between the rate of change of headlights, so it is fairly easy to estimate distance & speed.

On a motorcycle with one headlight at night, only light intensity (how bright the light is to your eye) and change in light intensity can be used to estimate change in speed.

This is very difficult for MANY reasons
                * The brightness of headlights varies greatly
                * The aim of the headlight affects the brightness perceived by drivers
                * The slope of the road affects perceived brightness
And the most impressive:
                * To estimate velocity, a driver must estimate by watching the change in intensity of a light.

Change in intensity has an exponential relationship to distance, not a linear relationship that holds for watching the angle between two headlights.

This means that to have adequate light intensity to judge distance, the motorcyclist is probably already too close to the driver to prevent an accident.

The next major point is Daylight Perception

The width of a 1975 ford mustang is 72 inches. A person with 20/20 vision can read 4-point font @12in and can recognize a 72-in wide car when the car is 1,375 feet away, which is ~1/4 mile. 

The width of a motorcycle is ~20 inches. A person w/ 20/20 vision can recognize the motorcycle when it is only 573 feet away.

It is legal to drive with 20/50 vision which means that the driver will recognize a 72-in wide car at 550 feet away and a motorcycle at 229 feet away.

A driver with 20/50 starting from a stop and making a left hand turn will have adequate time to react to a car at 550 ft away as long as it is moving less or equal to 147ft/s or 100mph. However the same driver with 20/50 vision starting from a stop and making a left hand turn will have adequate time to react to a 30-in wide motorcycle at 229 feet away if the motorcycle were moving less than 61ft/s (42mph).

Typical hard braking deceleration for a motorcycle was 16ft/s/s, meaning a motorcycle needs 2.76 seconds to come to a complete stop from an initial speed of 42mph, greatly limiting the amount of time to make an evasive move from a car.

Now a more contentious point supported by engineering is that motorcycles stop more slowly than cars. This makes sense insofar as the WEIGHT of a vehicle is NOT important when calculating the conversion of kinetic energy from velocity into the energy dissipated by friction
KE = 1/2 mv^2 = weight/2g*v^2 with g being the force of gravity
Energy dissipated by friction = weight(w) x distance(d) x coefficient of friction(f)
therefore v^2 = 2g*d*f with the weight cancelling out

However, maybe braking systems, etc have developed since this book and other forensic engineering books were published.

The author of my new book leaves us with the thought that maybe as the width of a car is 2.5x greater than a motorcycle, this explains that there are 2.6 times greater motorcycle accidents than cars.

I hope that my biker friends (including cyclists) can use this knowledge to keep themselves safe from accidents! 

Remember, it is difficult to impossible to judge distance and speed from a headlight. It is much more difficult to see a narrow biker than to see a wide car.

Best wishes! For more Forensic Engineering fun check out this book, my main reference for all of the above. I HIGHLY HIGHLY recommend it!!