Physics teacher "weighing in" here. I always thought that part of the danger was how high your head was off the ground, so after the last few posts I was curious. I did the math using acceleration due to gravity. If we are on road bikes, chances are our feet aren't flat, so I worked it for our heads being 6' above the ground. If we were to fall standing still with no interference, our heads would hit the ground at 13.4mph. Not sure how fast you guys can run, but I certainly can't reach that speed. If I ran as fast as I could and hit a low tree branch at my fastest, I'm sure there would be plenty of damage! Oh, better yet, reach that velocity on your bike and see if you'd want to hit the branch then! (For everyone who has a reasonable math system, I used 1.83 meters and it was 21.6km/hr. I actually used the metric and converted it to miles since metric is so much easier. But that's another debate...)

Since you would be more upright on a leisurely ride, seems the "slow" argument isn't a very good one.

Here are some velocities more for various heights.

5.5' = 12.8mph = 20.6 km/hr
5' = 12.1mph = 19.5km/h

Okay, school doesn't start until tomorrow...This is too much too early. I'm goin' ridin'!