This happened to me fifty plus years ago at a county fair.
A full field of pacers behind the starting gate, half way down the stretch, and headed to the start.
The ambulance is parked in the infield just past the judges stand.
The ambulance gets a call, drives out on to the race track, and heads straight towards the oncoming starting gate and full field of horses.
Lights and siren on.
No one hurt, but boy did we scatter!