The worst experience I ever had was with a motorcycle police officer zooming past me.
My friends and I were standing in the middle of a paved road on horseback waiting for the parade to start. So all the parade entries, ourselves included, are in the middle of the road. Well, there was a legitimate emergency that we didn't know about at the time, but the police officer zoomed, and I mean zooooooomed right past us riders. I was on my best, horse, but still, I couldn't believe that he would go past us so fast like that, on a motorcycle of all things.
Motorcycles can be scary for horses as it is, but to speed past an entire group of horses!??? We were lucky there weren't horses and riders spattered all over the pavement that day.
What good does it do to get to an accident quickly if you are going to injure other people in the process?
There was another incident (but I wasn't on horseback) where a firetruck came speeding down our dirt road, down the hill, through an intersection and on to where-ever it was going at a really fast speed without even pausing or slowing down! I was like OMG, what if a car was coming, that firetruck would have spun out in the gravel trying to avoid them. VERY scary how reckless the fire department can be!