To have reliable timing with the vehicles,
a) on start game activate all areas
b) on event of first area humans spawn, add the panhards to that same event, with the timer you want
the way this unfolds -
nothing happens until humans check off and enter the first trigger area, which is active as soon as the map loads (<event name="start_game"> happens immediately, even without players on the server). Once triggered, the script calls the event for the first area. There you usually spawn only humans, but now add the panhard events to that with their own start time. They will spawn/start rolling exactly at the time specified after we spawn.
I usually have a dedicated vehicle area to trigger those at slightly more random times and since I change the number and chance of vehicle spawns based on player count. I count players and then trigger different events, the more players the higher the chance for more vehicles.