I just wanted to clarify that I think that driving has the POTENTIAL to be more dangerous than riding.
The video that I posted is one that I accidentally viewed soon after training my horse to drive. Boy, did it scare the crap out of me! I hadn't thought much about the risk of driving, at least not in that it differed much from riding but this video showed me that it does.
There's so much more to account for (a cart, possible passengers, limited vision by the horse due to wearing blinders, less contact ect.).
That being said, I think that the danger involved in driving correlates directly with the disposition and training of your horse. My horse is dead calm, super steady and as close to literally bombproof as they get. I feel just as safe driving him as I do riding him. I think that driving is a blast and I wouldn't want to deter anyone from trying it. Just make sure you have a very trustworthy horse and a knowledgeable trainer to guide you.