Updated: Aug 21, 2020
I was riding my bike on the driveway across a pedestrian path. To cross the busy road I checked traffic on my left only: severe hailstorm was striking into my face from the right. Suddenly I found myself and a teenager girl on the ground. I jumped up to help her, mumbling my sorry, and offering to help but she, without a word or even look at me, picked up her phone, resumed texting and walk away. Embarrassed, I mount on my bike and rode home. If I were driving even at the same walking speed, my car would’ve thrown the girl under traffic with horrible consequences. In a collision between bikers and pedestrians, they don’t bounce from each other but just fall down. It's the conservation of momentum. 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 crumpling anything). A video from dashcam posted on Twitter demonstrated this classic mechanics.
The severity of road crashes depends on many factors but defining ones are impact speeds and masses of collided bodies. The 2nd factor, particularly at relatively low speeds, is the size and load per wheel of the vehicles.
Speed is the first to limit 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 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 cars are phased out by FELAs, Vision Zero will be achieved without even limiting speeds by 30kph. In 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 here 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 fall and hit the road or road’s entourage (meter, lamp-posts, curbstone, etc.). Running over is likelier for smaller victims (pet or child), high bumper or ground clearance e.g. SUV, light trucks.
As for running over, being not a forensic or traumatology guy, I just rewrite two cases from the mentioned article for lay-people like me. I don’t use pictures related to our crashes and rely on a vivid imagination of my reader to make my point to that lawyers and authorities who revise personal vehicles’ safety regulations and to categorize vehicles for taxation that would cover societal material & health costs of road crashes.
The first case: A man's, 48, thigh was bone-fractured and puncture-wounded by a car’s wheel. After fixing the bone, damage to soft tissue demanded so huge excision that in lack of antibiotics in early 1950-s, 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: After run over his foot by a van, a boy, 4 y.o., sustained a big flap of skin peeled off from the big toe to the top of the thigh. Amazingly, no bone fracture x-rayed but skin grafting was necessary and it took three months to heal. By the way, on the mentioned vid, a boy, after bouncing from car, jumped up just ran away. It doesn't guarantee from a fatal internal hemorrhage or organs' rapture.
Generally, the witnesses of similar road accidents describe the run-over as a victim was either car pulled up with one wheel trapping the body and might be reversed to release it; or the victim was flung off, skidding along the ground.
It seems the tire grips the skin and breaks its connection with deep fascia before stretched skin tears on the site of the limp that is not jammed between tire and road.
From the four factors named as determinants, I point out the top two: 1) it's the weight of a vehicle that inflicts the injury, still uniformly moving vehicle may only bruise a victim 2) when a heavy vehicle's speed is low, the torque will be significant, which makes the injury much more severe.
When a wheel, particularly driving or braking, runs over a limb, the wheel's load and torque do only matter. FELA’s per-wheel-load is 30-50kg, most cars have it tenfold. For example, if someone steps fully on your foot in an over-crowded bus with a 30 kg load, to his "Sorry", you reply, "Don’t mention it." When the 500kg-loaded SUV wheel crushes 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. The time of running over a foot at 10mph is about 0.02sec. Diameters are similar, thus shear forces acting on your foot for 0.02sec are 100N and 1000N respectively.
In case of a foot run-over by FELA, the skin’s flap won’t happen otherwise it would be described in a cyclist’s run-over pedestrian’s foot.
FELA wheel unsprung mass is ~1.5kg, cars wheels' is about 45kg. Dynamic pressures on your foot are 900N and 27,000N accordingly. 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 about the same (oppose a velomobile or road bike, FELA has big MTB tires). But FELA applies this pressure on the patch of one-tenth of a car's so traumatic effect will be even much less severe (due to distribution of pointed pressure in surrounding tissues) than from a car by a factor of ten. The driving or braking car wheel literally grinds delicate bones of a foot, not so in case of FELA’s wheel.
As we can see, of doubled average bike speed, FELA still can 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 case of a car that means losing your limp if you are far beyond childhood recovery capacities or within 24 hours after accident 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.
It’s 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.
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