That's an interesting question. The speed you can maintain while bike messengering depends on a variety of factors, such as the terrain, traffic conditions, and the weight of the package you're carrying. On a flat, smooth surface with minimal traffic, you might be able to sustain speeds of 15-20 mph. However, in hilly areas or heavy traffic, it could be significantly slower.
As for the conditions, riding in inclement weather like rain or snow can certainly slow you down, as can strong headwinds or tailwinds. Ultimately, the key is to maintain a consistent, efficient pace while staying safe and adhering to traffic laws.
I'm curious, what kind of speeds have you been able to achieve in your city? Have you found any particular routes or techniques that help you maintain a faster average speed?