I can actually tell you what this would be like as we do this with a Land Rover Defender. If you get much above 20 mph (30 kph) the sleds are getting air time over any bumps and get pretty scary. Wouldn't go much over 12-15 mph with kids in the sleds.
We are on private land with no traffic, so there's no reason to have to brake, but you're absolutely right that you can't brake otherwise the sleds will soon run underneath the back bumper, even though we are 3-4 times slower and use ropes twice that length, you have to back off the speed gently and let the sleds come to a natural stop.
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