Guineas roam, it's just their nature. Why they go in the road is questionable, but usually something attracts them... like other poultry or livestock across the road, good nesting areas over there, people/kids walking or riding bikes by etc. Is their coop situated close to the road? If so they may be part of the problem.
Short of babysitting and correcting/harassing them every time they head for the road and making them realize it's not as much fun as they think, they will most likely keep going into the road. Fences usually don't stop Guineas, but they do cut down on the majority of the wanderers, and also provide a visual boundary for the birds that they can be taught to respect with constant correction, but you have to be consistent and be ready to chase a lot of birds back in for a while until they learn. Some people hang shiny spooky things in areas they do not want their birds to go, some use the motion activated sprinklers.
You could also try penning them up for a week then only letting them out when you are around to keep an eye on them and herd them away from the road for a while... in hopes of reconditioning them to stay away from the road. Bad habits are hard to break with Guineas.
Good luck, hope you don't lose any more.