Your average casual cyclist averages something like 50 watts (and
that’s probably on the high side.) Even if 100% of this power was
saved (i.e. the cyclists rode nowhere) with 100% efficiency
(impossible), it would take 300 cyclists to power *one* car for a
similar amount of time (assuming that the car requires 20
horsepower average, which is just a guess.) As for the actual
energy recovered, 10% of your energy going into braking and
recovering 50% of that seems far more likely — so you’d need
6000 cyclists riding for one hour (or some combination thereof)
to power that one car for an hour. All that to save one or two
gallons of gas?

Storing energy lost to braking makes sense, but it would make
more sense if you could actually use it on the bicycle, rather
than trying to store it for later to power a *bus*. If the bike
would store energy when you used your brake, and then use that
energy to help you go again — THAT would be good.

Also consider how cheap 50 watts is. At $0.20/kWhr (a bit on the
high side), each cyclist is making one cent an hour — again,
assuming 100% power being saved (i.e. they’re not going
anywhere), 100% efficiency. That won’t even come close to paying
for the maintenance on the energy-saving system, let alone the
system itself. It would be far more cost effective to just put
some solar cells on the roof somewhere. Perhaps on the roof of
the bus itself? (Though the energy created by that even under
ideal conditions would be insignificant compared to the needs of
the bus — but covering the roof with solar panels would still
give your bus at least a hundred cyclists worth of `recovered
braking energy’ in bright sunlight.)

A person on a bicycle just doesn’t produce a lot of power. A lot of people on a lot of bicycles could conceivably generate enough to be useful, but the cost of the hardware required to collect and distribute this would be many many times that of the power generated.