Increasing the number of traffic lights on the most used route between the two destinations will increase the amount of time it will take to reach to the other destination, as each traffic light will cause a delay. Therefore, the increasing number of traffic lights are causing an increase in the average time to travel to the other destination.
In this case the correlation is most likely a casuation as the increase in the number of traffic lights is causing an increase in the average driving time between the two destinations.
