This is not "synchronous detection" in the way I meant it. I don't mean this as a put-down, but it may be instructive to Google "lock-in amplifier," a term I mentioned in my previous post.
All of your other concerns are addressed by appropriate choice of PRN code(s). Additional vehicles, which operate without the need for coordination, merely raise the noise floor. They do not "jam" each other. It should be obvious that a minimum S/N ratio is required for the LIDAR system to work and further that an arbitrarily-high number of LIDAR transmitters therefore cannot coexist. However, it is far from obvious (to me, at least without learning more about LIDAR and plugging in some numbers to a model) that a congested freeway of driverless cars would have "too many" LIDAR transmitters.