That's indeed the average adult walking speed.

Have you ever measured how fast you walk? I walk a couple miles every morning at an okay pace with my dog and I still average about 17 minutes a mile.

Factor in stop lights and other holdups, sounds like a good estimate to me. Now let us be generous and say 15min, thats still an extra 30min thus TulliusCicero's point still stands.

If I push it, I can get to my closest bus stop (~1 mile away) in 15 minutes (assuming no waits for traffic, but my legs will be tired. On my bike, it's closer to 5 minutes, less if I push it. If I'm considering taking the bus, you can bet I'll need to factor in getting to the stop. As it stands, taking the bus to my job takes ~40 minutes if I take my bike, 50-60 minutes if I walk, which is really bad compared to my 20 minute commute by car.

I'm a fast walker, but I definitely wouldn't consider taking the bus if it meant walking everyday.

Figuring in traffic lights, this is pretty close, even as a fast walker.

We can't all be 23 year old marathon runners who shower and change at work.