Bikes should be an integral part of transportation in the city.  Instead, because of insensitive drivers and car doors, many travel by car to a more suitable  location.  Sort of negates the environmental benefits.

If you pedaled  at 20 mph off  the end of a truck that was traveling forward at 25 mph, you would be moving at 5mph in reverse.  I would imagine it wouldn't be very good for the mechanics of the cycle, also very hard to control .