The effect of overtaking lanes on average speed on Finnish main roads

Finland is a relatively sparsely populated country with long distances between cities and towns. Driving a car is the most common way to traverse these distances and most of the road network in Finland, particularly in rural areas, consists of two-lane roads – meaning one driving lane in each direction. On a two-lane road, a single slower vehicle in front can cause delay in travel time. As a solution for this, opportunities to overtake slower vehicles are often provided in the form of overtaking zones or overtaking lanes. This study aims to provide insights into how overtaking lane segments affect the average speed of traffic, and moreover, examines the difference in average speed on road segments before and after an overtaking lane segment. This is done by looking into 8 cases of overtaking lane segments on Finnish main roads, with segments lengths ranging from 11 to 19 km. The speed data was sourced from the car GPS provider TomTom, and the road segments were divided into different zones according to speed limits and overtaking lane segments. The results indicate that overtaking lane segments can slightly increase the average speed of traffic by 1–4 km/h over stretches longer than 7 km, if the conditions are suitable. Key factors hindering this were identified to be approaching urban environment and the order of forced lower speed limit zones. Interestingly, in some cases, a lower speed limit zone following an overtaking lane segment did not eliminate the speed buff effect. Approaching urban environment refers to a city centrum ahead of the driver. While the data used in this study does not allow precise definition, visual cues and observations such as buildings, well-maintained surroundings and high-traffic at-grade intersections likely con-tribute to this. Seasonal cycle was found to affect average speeds before, during and after overtaking lane segments, but not the rela-tive changes in speed. If an overtaking lane segment caused a speed buff during summer, this was always very closely mirrored in the winter data as well and vice versa. The same trend mostly applied to different times of a day. The length of an overtaking lane segment or a specific overtaking lane layout was not found to have a significant effect on the occurrence of the speed buff. During the overtaking lane segment, driving above the posted speed limit seemed to be standard during summer. In general, speed cameras always greatly reduced average speeds, but only very briefly. The effect was very point-like, and typically did not persist longer than 400–500 m.