I would first say “know your horse” before going out alone with him as a single. A young horse, a new-to-driving horse, means you should take extra precautions, get some help to just ride along, until horse gets time and miles on him.
An experienced driving horse? Obedient, fairly spook-proof and traffic safe? I see no issue with going out alone driving. You take common precautions, carry the cell phone ON YOU, have your spares, halter and lead, in the vehicle for safety.
As many have said, if you don’t go alone, you never get to go at all. Horse tends to be YOUR toy, spending time using him, is YOUR enjoyment, ridden or driven. So going out alone is pretty common among horse folks I know. Great if you have a family member to share with, just don’t count on them always being around.
I did the single horse driving way back, went out and did MANY miles of fun with each of my girls. I did the conditioning, Husband did the detail stuff, since he was the Driver at the CDEs. Then we went to Pairs, Fours, which he likes better, and I don’t drive them hardly at all. Hitched that way, they are HIS horses, tuned to his voice and hands. I ride with him doing the Multiples, or am there watching him do ring work. Just not safe for one person to be hopping down and fixing stuff, then getting back up, takes too long. I can reset reins in 30 seconds, tighten a strap, get tail hair unstuck from harness, then get him back on his way whether on the road or in the ring.
Go with what feels comfortable to YOU with THIS horse and where you drive. Some horses or locations to drive are just SCARY, so you don’t go out alone. Change the horse or road you drive, no issue going out then. I knew my girls, raised and trained them, rode them, so we were very comfortable in all settings I took them to. Local roads are busy, but very visible, with MOST cars and traffic being careful around you, so are easy to drive on for long distances to get that conditioning they needed. Other places are blessed with nice back roads, so driving is a lot of fun.
One last note, is to NEVER consider muscling a Mini around. He needs to be TRAINED, have the depth of skills that ANY equine should have for driving. Little size is DECEIVING, because 300 pounds of fighting Mini is going to haul even LARGE MEN right down the lane. Seen it happen more than one time!! Minis are so accepting, until they are NOT, so they are big foolers in depth of training. Folks just put the harness on, ground drive a day or so, hitch the cart, go to driving their Mini!! He then has no “basics” to use when a bad situation happens, no idea how to react properly. You CAN NOT hold him if he chooses to leave.
So do all the steps with a Mini, that you would use training a 15H light horse to drive, along with TIME and repeating those steps to make them stick in his head, even though Mini NEVER reacts to anything during the training. Minis can be quite wonderful, but they are NOT “born broke”, so you have to train them. They are just very “accepting” until badly frightened enough to take off. They are NOT dogs, you will have way more than you can handle if he fights you even as small as they can be.