Operant conditioning. Running in the road and getting spanked are both tangible experiences. Being hit by a car is entirely hypothetical until it actually happens. Learning the association "running in the road hurts" or "running in the road makes me sad" requires less much less cognitive development than understanding the hypothetical "if I run in the road then I might get hit by a car, and getting hit by a car will hurt". The latter requires a degree of imagination that many young children have not yet developed.

I'm not saying with any confidence that spanking is necessary in these circumstances, only that it may be justifiable given the risks involved. You need some sort of strong negative stimulus. It's not nice, but neither is being hit by a car.

https://en.wikipedia.org/wiki/Operant_conditioning