Tying the dog to the bakkie is dangerous. Ropes and leashes become nooses if the dog should be jolted out of the bakkie.
If your bakkie hits a bump or swerves to avoid an obstacle or if you step on the brakes suddenly, a dog riding in the bakkie bed can easily be thrown out on to the road or into the hard surface of the bakkie bed sides. If being hurled on to the street or into solid metal does not kill or injure your dog, being struck by another vehicle probably will. And, in trying not to hit a fallen dog, another driver may cause an accident. In fact, motorists have been killed trying to avoid a dog thrown from a truck.
It is believed that hundreds if not thousands of dogs die this way each year. Every year vet practice see dogs that have been seriously injured from jumping out of the back of a moving bakkie. It happens at high speeds or very slow speeds. Some have been riding in the back for years before suddenly deciding to jump out.
Your dog isn't necessarily safe even if he/she does manage to stay in the back of the truck. A few dogs do love the feeling of wind blowing past their ears at 60 mph, but that wind can seriously irritate mucous membranes and blow pieces of grit or bugs into the animal's eye. Debris may also lodge in the nasal passages or get sucked up into the windpipe.