For once I had a day off, lots of snow, no snow coming down, plenty of sunshine AND someone to take pictures(!) so I hitched Kody up to the sled I bought two years ago and tried it out. I had a singletree arrangement I made for my drag tire which I attached to the black plastic sled (yay, baling twine!) and off we went.
Long story short, the sled itself was fine and pretty comfy with newspaper padding to sit on, the arrangement from sled to singletree was fine, but the singletree was too close to his hocks and made him nervous. It didn't actually hit him while he was moving but when he stopped it brushed his hind legs and he didn't like that. After successfully driving in the arena for awhile we went out on the road where he took exception to our dog trying to race him and took off. I stayed in and kept the sled upright for quite awhile but eventually he swung me out towards a ditch and the edge of the sled caught the snow and flipped and I chose to let go of the reins rather than get dragged alongside it.
He wasn't scared or kicking at that point but without my weight holding the sled back it did start coming up and hitting his hind legs as he ran so by the time I followed his tracks to the fenceline of the boarding barn behind my neighbor's where he was patiently waiting, he was more than a mite touchy about that singletree being near his legs.
I held it up as we walked home and he was fine, and he was fine ground-driving afterwards.
I think the main safety things are to have something that won't flip when the horse goes sideways, be fairly close-coupled so they don't swing you into a tree or something but far enough back they can't kick you, and keep your center of gravity low. I was so glad I wasn't on my sled with runners! I would have flipped a dozen times before we even had the accident. I'd like to make a set of shafts for my plastic sled, maybe with a nice duga (an arch over the horse's shoulders) for looks. That would keep it back off of him and I need to either let out his traces, use baling twine on the ends to extend them or get rid of the singletree entirely and attach the twine directly to quick releases on the sled itself. Hopefully he'll still be willing to pull it!
The biggest thing I found, rather than it being hard work to pull a sled over 2 or 3" of snow, was the way the snow balled up in his hooves. A large part of his unhappiness today may have been because he was uncertain of his footing.
Leia