[Bob.Kerns]

My list of falls (in > 4000 miles) are in approximate order:

1. High centering traversing a rut.
2. Very steep hill covered in very loose dust. After the fall -- I tried to get up, and slid down the hill on my hands and knees. THAT hurt!
3. Knocked off a path and down an steep canal embankment by a falling bicyclist. Falling was deliberate -- it kept me out of the water.
4. Hitting the left wheel on a doorframe entering a bathroom with a pre-ADA narrow doorway and a crazed over-sexed door closer on steroids. Twice.
5. Climbing up a steep bit of slushy snow.
6. One wheel off the sidewalk onto slick wet grass to let pedestrians pass, attempting to turn back onto the sidewalk. The turn didn't happen with the expected timing, and I lost my balance (the only time I've lost my balance and fell, rather than the Segway losing the ability to balance. I've lost my balance -- but the Segway generally follows and gets back under, if you hang on for dear life!)

So high centering, losing traction, hitting something that blocks the forward motion, and falling bicyclists would be the primary dangers.

Tree roots can cause all four.