When I went to university, I was going there in rollers, there was a nice slope where I could go relatively fast, in the morning, almost no traffic, good visibility, relatively smooth surface. All it took was a small peeble to launch me a few meters (tens?) flying away. The fall was much more impressive than the damage, I consider myself lucky and was basically the last time I used them. I realized how dangerous it was in comparison to just biking.