I have to admit, they're pretty cool. But it has many flaws, IMO.
How do you know you're not forcing the horse to go x-amount mph? Like the car/machine-thingy is literally pushing the horse to go 45mph when it can only go 35mph comfortably (without a machine/car/thing)?
What happens if the horse stumbles and falls? Sure the car-thingy will shut off, but it wouldn't be able to come to a full/complete stop in a split second...That's enough time for the horse to end up under the car.
But I'd like to see where this ends up going in terms of training horses...In a way, its basically an advance hot walker.