I agree Roody

I used my MTB (with top gear of 46x13) on a metric century (actually 50 miles) yesterday - I brought it instead of my road bike for the triple crank as it was a hilly course.

However on flat sprint portions it was kind of a joke. I was better than 20 mph but nowhere near the 30+ one can get on a more aero road bike with 53x11 (not that I can get 30 but you get the point).

I rather wished I'd brought my road bike instead. The previous week I did part of the NYC Century and the mtb was fine for that. I managed to hit about 20mph into the wind along the shore rd in Brooklyn.

That said, I also agree the mtb's speeds are fine for me in urban commuting; in fact it works out better because in rush hour traffic I prefer to ignore the urge to speed up - that's a good way to get creamed at intersections or doors or potholes, etc. For my urban commute the MTB bike is more than sufficient.

You needn't go S-L-O-W when commuting to be safe but sane, assertive speed (emphasis on SANE) is good for safety. I prefer that and the better stopping power of MTB brakes vs. a road bike.