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How to Move Faster without Road Kills on Residential Streets

Updated: Feb 4

I was riding my bike on the driveway across a pedestrian path. To cross the busy road, I only checked traffic on my left: a severe hailstorm struck my face from the right. Suddenly I found myself and a teen girl on the ground. I jumped up to help her, mumbling my sorry and offering my help, but she, without a word or even looking at me, picked up her phone, resumed texting, and walked away. I mounted on my bike and rode home.

If I were driving even at the same walking speed, my car would’ve thrown the girl under heavy traffic: In a collision between bikers and pedestrians, they, as physics of comparable masses"prescribes", don’t bounce from each other but fall down. When a car hits a cyclist or a pedestrian, unless being run over, they are thrown off (at the doubled impact speed, assuming no energy is spent on crashing bones). A video from the dashcam posted on Twitter demonstrated this classic mechanics.


The severity of road crashes depends on many factors, but the most essential are impact speeds and collided bodies mass ratio. The third factor, particularly at relatively low speeds, is the vehicle's size and load per wheel.

Speed is the first that must be limited because impact energy is proportional to speed squared and to the collision probability (less time to react and harder to swerve). However, below, we consider the role of a vehicle mass, specifically the load on the driving wheel.


Friction Injuries Following Road Accidents, British Medical Journal, p. 262, Aug. 2, 1952 article shows that load per wheel is critical for crash severity on residential streets.

Since a car's per wheel load is ~10x FELA's, when FELAs phase out cars, Vision Zero will be achieved without limiting speeds by 30kph.

In the case of cars, the limits on residential streets are futility - a half-a-tonne rubber roller smashes any flesh at any speed.

As of secondary importance in case of hitting vulnerable street users, we don’t discuss a car’s safety design features but the car's size and weight.


Worth repeating: when a car runs into a pedestrian or cyclist, they either hit and thrown off or get run over. The hit-thrown-off scenario is likelier the bigger car or higher its hood. When it happens at relatively low speeds, severe head injury follows from a fall and a hit of the road or a meter, lamp-posts, curbstone, etc. Running over is likelier for smaller victims (a pet or child) by a vehicle with a high bumper or ground clearance, e.g., an SUV or truck.


As for running over, being not a forensic or traumatology guy, I rewrote two cases from the article mentioned for lay people like me. I don’t use pictures related to our crashes and rely on the vivid imagination of my reader to make my point that lawyers and authorities who revise personal vehicles’ safety regulations and categorize vehicles for taxation would cover societal material & health costs of road crashes.


The first case: A 48 y.o. man's thigh was bone-fractured and puncture-wounded by a car’s wheel. After fixing the bone, damage to soft tissue demanded such a huge excision that, due to a lack of antibiotics in the early 1950s, the man died four months later. Nowadays, even if he survived, he wouldn't have a stub of sufficient length to attach an artificial limb.


The second case: A van had run over the foot of a boy, four y.o. He sustained a big flap of skin peeled off from the big toe. Amazingly, no bone fracture x-ray showed, but skin grafting from the top of his thigh was necessary, which took three months to heal.

By the way, in the mentioned YouTube video, a boy jumped up after bouncing from the car and ran away. It doesn't guarantee that a fatal internal hemorrhage or organ rapture hadn't happened.


Generally, the witnesses of similar road run-over accidents describe it as either car pulled up the victim with one wheel trapping the body and might be reversed to release it; or the victim was flung off with consequent skidding along the ground.

It seems the tire grips the skin and breaks its connection with deep fascia before stretched skin tears on the limp side that is not jammed between the tire and the road.

From the four factors named as most essential in low-speed collisions, I point out the top two: 1) it's the weight of a vehicle that inflicts the severe secondary injury from fall and hit, while a slow, uniformly moving vehicle may only bruise a victim 2) a heavy vehicle driving wheel's significant torque makes the injury much more severe.


For the wheel running over a limb, the wheel's load and torque only matter. FELA’s per-wheel-load is 30-50kg, most cars have it tenfold. For example, when someone steps on your foot in an over-crowded bus and says, "Sorry", you reply, "Don’t mention it." When the 500kg-loaded SUV wheel crushes the delicate bone of your foot, I doubt your ability to say anything with a forgiving smile.


Next, FELA moving at usual for neighborhood 10-15mph requires a torque of about 10Nm. The 20x heavier car needs 200Nm. Running over a foot at ten mph lasts about 0.02 sec. Wheels' diameters are similar. Thus, shear forces acting on your foot for 0.02sec are 100N and 1000N, respectively.

In the case of a foot run-over by FELA, the skin’s flap won’t happen. Otherwise, it would be described when a cyclist runs over a pedestrian’s foot in numerous cases.

FELA wheel unsprung mass is ~1.5kg, and car wheels' is about 45kg. Dynamic pressures on your foot from those wheels are 900N and 27,000N, respectively. It's huge in both cases because of the assumption of absolute rigidity of both wheels and your foot. But with pneumatic tires, pressure on your foot equals the tires' air pressure, which in both cases is about the same (as opposed to a velomobile or road bike, FELA has big, much softer MTB tires). But FELA applies this pressure on the patch of one-tenth of a car's, so the traumatic effect will be even much less severe (due to the distribution of pointed pressure into the surrounding tissues) than from a car by a factor of ten. The driving or braking car wheel literally grinds the delicate bones of a foot, not so in the case of FELA’s wheel.


As we can see, with doubled average bike speed, FELA can still be the vehicle in urban areas where Vision Zero can come true without increasing trip time.

FELA's rolling over your limp is similar to someone's accidental step over in public transit, in the case of a car, that means losing your limp if you are far beyond childhood recovery capacities or within 24 hours after the accident, a micro-surgeon will be found, and your insurance covers a dozen hours of collecting debris of your many little bones into the a functional order.


We all have heard horrible cases when parents backing off a car from the driveway run over their child playing behind on asphalt. That kind of horror won’t happen in a neighborhood where cars are replaced with ultra-light and compact active personal vehicles – FELA.


Let's add a perspective and stress it again:

It’s the weight and size of a vehicle that must be taxed, not the size or power of its motor. And right after speed, the weight and size of the vehicle determine not only its danger but all the environmental externalities.


Help me to see the flaws in my reasoning above by relying on it in the box below.

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