You could consider using either transmission mediums that are more directional.  Infrared or laser signals tend to be very directional, and can be modulated to contain information, just like any radio frequency signal.

You can Contact a company call Opticom they make infrared devices that are mounted on traffic signals and are wired to the controller cabinet.  They are unidirectional and are used by fire and ambulance services to switch on the green in their direction of travel and to supply reds to all other directions. Hope this helps.

The first suggestion about using an IR or laser is the best so far. The problem using even a very narrow beam antenna and a radio frequency(RF) signal is reflections off the cars and other surfaces that may find their way into the undesired lanes. Take Police Radar for example. They use very directional and very narrow beam antennas and transmitters at microwave frequencies. These waves are reflected off winshields, fenders, wheels, etc so that your radar detector picks them up. The same thing willl happen to the IR or lasers as well, but very focused light does not scatter as much as RF so the chances would be lower. I assume you are considering this to make cars slow or stop automatically when lights change color. One way to accomplish this is to have each light either transmit a unique frequency, or a unique digital code that is received by a car which "knows" which direction it is traveling in (maybe by use of a GPS receiver?). The code would correlate with the direction of travel so the car would know when to slow or stop. Then you could use one omnidiectional antenna at each light.