If you rewatch the vid you'll see red and white "flags" on those obstacles. That helps the driver, but as you can see and hear from the announcer those drivers can make decisions based on time, speed, and the horse's ability. Plus, just like jumpers and cross-country horses the rider sees the course and walks the course. If you watch them, they are counting paces between the jumps based on how length of their horse's stride and ability.