To be clear, the streets I'm talking about are the typical larger, un-bike-friendly, street in a dense city - flat, four traffic lanes, two parking lanes, no bike lane, traffic lights every several blocks, cars moving 25 mph to 35 mph with the speed limit 25 mph.

On those streets, if I'm pedaling 25 mph, I'm moving about the speed of a slow car. At 20 mph I'm slower than any car, but the cars don't pass me as quickly (relative speed). My top speed during those commutes is about 28 mph, which I can only do for a short time, and at that speed I can change lanes and pass slower cars as if I were a small scooter or moped, and traffic seems to treat me like one of those - no-one tries to share my lane. Of course, in a few blocks I am back to "slow car or slower" speeds, with my heart dangling from my chest, dreaming of a Gruber Assist ( http://road.cc/content/blog/4155-ele...e-hidden-power ) and feeling dirty . . .

When I'm moving slower than cars, I ride to the right of the lane, just out of the door zone, and in stretches with no parked cars I ride further to the right if a car is bottled up behind me. I use a helmet mirror to monitor what is going on back there. The faster I'm going, the more of the lane I take.

Here is a way to think about it. You know when you're on a city street with a downhill grade, that makes riding 30+ mph easy? Do you feel safer, vis-a-vis traffic, at those speeds?

All of the above applies only on flat roads. For me, even a slight upgrade (not even a proper hill) completely changes the options. Do you crank up the effort to hold 20 mph? If the grade is only for a couple of blocks, sure, but I can't keep that effort up for long, and the harder I'm struggling, the less alertness and bike control I have. Do you drop it down to 15 mph? Now the cars are elbowing past you, mirrors whizzing by. Do you just bail to the parallel quiet street? That's probably wisest.

I was talking about this with some other commuters, and they pointed out that when riding fast, you might be more focused and alert, not daydreaming. I've also noticed that the faster you ride, the more green lights you catch. Makes sense if the signals are timed for the speed limit. On my experiment days, I seem to spend a lot of time at red lights. I don't like red lights - sitting there waiting to be rear-ended, then trying to clip in and accelerate with a car on your butt, plus yesterday I almost got right hooked by a commercial truck when one of those reds turned green. Feels safer to go through on green.

Note: I don't regularly commute on roads with typical car speeds of 50+ mph. I've ridden on those, and I agree whatever puny speed a cyclist can come up with makes no difference there. Just stay far to the right, watch your mirror, and be prepared to dive into the bushes.