One time we used 2-way radios, another time we used push to talk cell phones that a local wireless store gave us to use, the push to talk did not work so well.

Basically the lead car passenger would radio his location on the course to let the marshals on the upcoming section know they were coming and to hold traffic. If you can get police involved, it makes for dealing with cars a little easier, if you can have a cop car drive ahead of the lead official car it makes drivers a little more apt to pull over and get out of the way, but police cost money.

It's extremely important that you brief the marshals on how to handle irate drivers, we would hand out a flyer for the marshals to read with instructions on what to say, our race was in a pretty well traveled area and some drivers got pretty pissed about having to wait a few minutes, but you have to have those marshals be wiling to do almost anything necessary to keep the cars from pulling out when the race is coming or it could get real ugly with a car coming on to the course.

Our race only lasted 2 years cause the local a-hole residents caused such a stink about being delayed in getting to the mall, mcdonalds or wherever else they were going that the township would not allow us to hold the race anymore. That's why in my area there are way more crits then RR's, it's much easier to hold a corporate park crit then to put on a decent road race.

Good luck with the race.