Are you worried about

Bolting horses and ponies?

Anyone who works with ponies and horses knows that bolting is the biggest single risk. In fact anyone who knows the English language is aware that people don't have a word "bolting" unless they have the problem "bolting". 

When ANYTHING goes wrong, I can  release the pony, instantly.

 Years of watching the vehicles in the workshop show that without a pony attached, the vehicle just sits there. 

A stationary vehicle, with automatically applied brakes is NOT a risk.  Releasing the pony does exactly what the UK Provision and Use of Work Equipment Regulations  (PUWER 1998) requires. 

I must be able to isolate my work equipment, the Obamobile 3, from all it's sources of energy, also known as Obama the pony. The Instant Pony Release System means  Obamobiles appear to be the only vehicles that comply with PUWER 1998.

We comply because we work in Scotland and Nature.Scot, aka Scottish Natural Heritage  require all horse drawn equipment to comply with PUWER 1998.

Loose Ponies

All the horsey people say, "But you've got a loose pony!", and I can't deny it.  But why worry. On Dartmoor and in the New Forest, wild ponies are marketed as a tourist attraction, (see https://visitdartmoor.co.uk/things-to-do/dartmoor-ponies/ and https://www.thenewforest.co.uk/explore/wildlife-and-nature/ponies/ ) Tourist attractions are not dangerous. Safety is a pretty major factor in becoming a tourist attraction.

To understand pony behaviour under stress, think Zebra. This is the closest genuinely wild living relative.  A  Zebra's big fear is predators, particularly lions. If a lion attacks you run first, think later. Running instantly means you aren't where it aimed to kill you. Once you are moving flat out you have time to think about direction.

If a pony is pulling a vehicle, when it starts running, the vehicle chases it. An animal whose big fear is being chased, is now being chased. If you can release the vehicle Instantly, safely and reliably, the pony isn't being chased. And it stops when it is in open space. 

If the pony is being chased by a dog, it will keep running, but remember, it is taking the dangerous dog away from you, and the people you are working with.

Follow the link for more details on pony behaviour 

Safety is a massive subject, and I discuss it further in Obamobile Vehicles and in more pages here. But the simplest and most effective safety device when a pony is doing something stupid, is to let the pony go and be stupid somewhere else.