A wire in the road can't handle stuff like a pedestrian. Pedestrians normally aren't in highways but sometimes are after break-downs, accidents, construction, etc. Lane-following is close to solved on the highway, it is all that other stuff and more that is an issue. A wire could still help, but you'd still need a complex system or significantly more infrastructure than just a wire (maybe caged barriers over the lane in a way that doesn't cause issues if there is a fire, and more).