On 24 Jan 2011 20:06:01 GMT, John Doe wrote:

Specifically... What is the approximate required horsepower for a DC
gearmotor to push a bicycle at 20 mph or less on terrain less hilly
than San Francisco.

Thanks.
Well, the legal limit in Canada is 350 watts - that's roughly 1/4 HP
in real power output - and is marginal if you do not pedal. I'd say
1/2 HP would be pretty reasonable, and 1 HP much more than adequate.