Dynamic speed limits, change blindness and situation awareness

Dynamic speed limits are one of the latest traffic management pilot in the Netherlands. Being dynamic they can vary during the day. Previous studies showed difficulties with noticing small changes, also called change blindness. These change blindness studies have mainly been conducted using static scenes or changing irrelevant objects or changing objects that are perceived as static. But what happens if the information changed is relevant for the task, can be perceived as dynamic and can be seen continuously in a dynamic scene?On a Dutch freeway dynamic speed limits were tested that increased from 100 km/u to 120 km/u at moderate traffic conditions. They were shown on rotation signs next to the road and varied several times a day. Loop data was used to measure average speeds of passing cars every minute and compared with the dynamic speed limits displayed. To minimize side-effects only data from speed limit increases for over an hour were used.The average speed before a switch is 107,7 km/u, meaning that drivers are driving on the pilot route for approximately three minutes before passing the detector loop. Drivers passing the loop did not react immediately on an increase in speed limit. On average it took three to four minutes before car drivers increased their speed. After that driving speed starts to increase significantly. Drivers in the middle and left lane are quicker to act on new speed limits than drivers on the right lane.Three minutes of non-response equals the amount of time it would take the last car driver that could have seen a 100 km/u sign to pass the detector loop. This may mean change blindness is involved. Drivers that might still have seen 100 km/u signs before the switch do not act upon the new speed limit. Subsequent drivers who never saw the 100 km/u sign do; they never saw information that changed. The quicker reaction of drivers in the middle and left lane maybe due to their more active driving style. These drivers are generally busy overtaking other vehicles. This increases situation awareness making them more sensitive to changes in their environment.