I suspect that by "synchronized" they simply mean "coordinated" -- that they are automatically controlled by a computer that attempts to optimize for flow. So

I suspect that by "synchronized" they simply mean "coordinated" -- that they are automatically controlled by a computer that attempts to optimize for flow. So for example, an intersection will be kept open in one direction as long as there are no crossing traffic. And if the same is the true about the next intersection, the light will be green there too; based on the sensors, the system can gauge the amount of traffic going towards the next intersection, and prioritize the flow. So if there are a lot of cars going towards an intersection, and a single car arrives that needs to cross their lane, the system may favour the group and prolong the green light. In that sense it's synchronization, as "adjacent" lights will be considered by the algorithm.