The traffic lights change in turns, depending if they are pointing more towards North/South or more towards West/East. No matter where in the map the light is located, they still change at the same time depending on their direction (tested in Map Editor). So basically there are 2 different sets of traffic lights that would need to be synced.
Here is the timeline for the loop:
0:00 - Set 1 turns green / Set 2 stays red
0:12 - Set 1 turns red / Set 2 turns green
0:22 - Set 2 turns red / Set 1 stays red
0:31 - Set 1 turns green / Set 2 stays red
Total: 31 seconds
* Set 1 = Pointing towards North/South
* Set 2 = Pointing towards West/East
Both of the lights stay red for the last 9 seconds of the loop, because that's the only period in the loop when the pedestrian lights for both sets turn white.
However, at the moment it seems that the sync of the traffic lights between two players depends on their FPS.