Some Gym equipment has magnetic resistance
Fillthe cup,
Some gym equipment has magnetic resistance already-I have a semi recumbant stationary"bike" that has this kind of setup. It is about 90% of a generator, so I can't think of any reason to not make it a full generator..Now we don't produce a lot of power.I would guess most of the time our/my output is no more than 1/15 HP-or about 50Watt Hrs per hour.This is about 1/2 cents worth of power per hour.This much power could push a fairly light, aerodynamic car about 400 yards at 20mph.Still every little bit helps-might as well get something other than sweat for an hours worth of pedaling.
There have been generator driven lights on bikes ever since I was a kid-1950's. You must have seen them-you pull a lever and a little knurled wheel moves onto the tire and is spun-it direct drives a generator which powered a really dim(by current standards) headlight..
No reason a device like this couldn't be used as a brake.The only downsides would be weight-generator/battery/electric motor and cost.
There are currently5 passenger 3000 lb hybrid cars that get an honest 45 mpg in pure city driving. I don't see any reason a 30 mph electric/hybrid/bike/Moped(150 lbs with lights,enclosed fairing,cockpit etc) maybe a semi recumbent couldn't get 40 miles on 10 cents worth of electricity-(1000 watt Hrs).It would be like getting 800mpg.
Oh well, running a bit wide here.Luck,
Charlie