In many years road trains has been used internationally, but in 2008 were the first trials with road trains initiated on selected parts of the Danish national road network. The period of the trial was at first nominated to three years, but was extended several times. The trial period is now prolonged to 2030, which in reality makes it a permanent arrangement. In the initial stages towards the trial period the transport industry expressed their needs for road trains, which led to the ongoing trial.
Road trains in Denmark are up to 25,25 m long, and therefore it has been necessary to rebuild state roads and selected municipal roads, were these road trains are allowed, to ensure suffi-cient space. The rebuilding is mainly required in the nodes, which involve design chances of the nodes. Around 150 locations are in total rebuild so far, where a considerable part are rounda-bouts, to ensure sufficient space for the road trains. This project focus on single-lane rounda-bouts, where the central island is reduced, and the reduced area is added to the lane. The re-building of the single-lane roundabout makes the turning radius bigger, because of the increas-ing width of the lane, which could allow especially private cars to increase their speed through the roundabout. The hypothesis is therefore, that the rebuilding of the single lane roundabouts will increase the speed noticeably through the roundabouts. An increase in the speed could led to an increase in the accident risk and an additionally increase in speed variation, which is a normally outcome of an increasing speed level.
To confirm the hypothesis is two studies made: A before and after study in the same rounda-bout and a with and without study in two almost identical roundabouts. The three roundabouts are all outside urban areas, have an AADT of 4.000 to 8.000, and the roads leading up to the roundabout have a 80 km/h speed limit. For each of the three roundabout approximately one week of video registration were recorded. The study only focus on private cars driving under free flow conditions, and the study is based on 100 cars per week per location, so 400 cars in total for the study. The speed through the roundabouts are measured from two timestamps between two appointed location in the roundabout.
The result of the study were, that the speed in the before and after study are almost 28 km/h and 37 km/h respectively, and the speed in the with and without study are almost 32 km/h and 33 km/t respectively. The results of the study are tested for the same mean value using statistical analysis. The combinations that were tested for same mean value are before and after, with and without, before and with, before and without, after and with, after and without, and finally before/with and after/without. The result of the statistical analysis are, that none of the combination except the with and without have same mean value, which shows, that the speed has increase since the rebuild of the roundabouts. An increase in the speed in rebuild roundabouts might reduce the traffic safety and increase the risk of accidents, according to the power model.