Public road dog track system

Anyone who has had a bouncy dog, a car, a quiet road through a field and a cold, rainy day will have tried this: Dog out of the car, twenty minute drive, dog in the car, home for tea. Mine does about 30mph (the dog that is - the car will go marginally quicker) and loves trying to catch the car. If you live in the city, you will hopefully not have tried it, but you could try this:

Box off the area of road directly behind a pickup using tubular steel: weld two sections either side of the pickup extending back about 3m and a crossbar holding the number plate and maybe a red warning triangle on each corner for luck. This will protect the dog's running space from vehicles. Install a springloaded swingarm with a catch on it into the back of the pickup. Harness the dog (a harness with wide, comfy straps would be best) and tether the harness to the end of the swingarm, leaving a little slack but not enough to allow the dog outside his 'safe area'.

You can now drive to the shops or wherever you're going and the dog can run after the car and get all the excercise he wants. If you drive too fast or he tires and slows down, the tension on the tether will trigger the swingarm catch which will pull him up off the road and into the back of the pickup where his bed and a bowl of water is waiting. I'm sure hanging a sausage off the back would increase the fun, but I don't think my dog would need any encouragement!