Traffic managed? Why not all drivers can adhere to changes in speed limit signals over the road

Perceiving changes in variable speed limits is a prerequisite for speed limit compliance. However, under certain circumstances our ability as drivers to detect such changes around us is restrained. Variable speed limits embody traffic management information which drivers regard as very important and they will therefore be perceived more often. The present study focussed on the detectability of an expected change on a familiar route and compared two traffic management approaches – only display information when necessary versus display information continually – in terms of the ease with which drivers perceive changes in this type of traffic management information. In a 2x3 design, change detection for variable speed limits was measured for information addition and information change under three conditions of information discriminability. Participants were repeatedly shown videos of a single motorway to familiarise them with the route. Although all drivers were aware of an imminent change and (almost) all expected the variable speed limits to change, 5.2% still failed to detect when the speed limits had actually changed. Though this number seems small, the absolute number of cars on motorways missing the change is unacceptable. The results are discussed in relation to detectability of, recollection of, and expectations about the new speed limit. This study provides insight into change detection failure for dynamic traffic management information and possible countermeasures.