Answer
The transition from open roads to tunnels causes a transition in illuminance circumstances, in particular in the daytime. It is important to realize a gradual transition at the entrance and exit of a tunnel; the human eye needs several seconds to adjust to the change of situation. If the illuminance transition is too abrupt, road users are unable to discern details for a few seconds and that may be enough to distract them (see the archived factsheet The road safety of motorway tunnels).