RESEARCH ARTICLES CURRENT SCIENCE, VOL. 120, NO. 4, 25 FEBRUARY 2021 699 *For correspondence. (e-mail: vvasudevan@alaska.edu) An empirical study on the influence of on-road static obstacles on driver behaviour Bhupali Dutta 1 and Vinod Vasudevan 2, * 1 Department of Civil Engineering, Indian Institute of Technology, Kanpur 208 016, India 2 University of Alaska, Anchorage, 2900 Spirit Drive, AK 99508, USA Understanding the influence of static obstacles on driver behaviour is important because drivers’ instinctual reactions may pose serious hazards to safe- ty and mobility. The effects of road geometry, road- side infrastructure and static objects near the road on driver behaviour have been studied extensively. How- ever, no studies discuss the influence of two variables that are to likely to affect driver behaviour: obstacle size (width and length) and the side on which the stat- ic obstacle is present (driver’s side or passenger’s side). Controlled experiments were conducted using an instrumented vehicle to collect instantaneous driver behaviour data in the presence of on-road static obstacles. The results show that the side on which the obstacle is present influences the lateral gap. The results also show that obstacle size, both in terms of width and length, affects driving speed and the lateral gap. The results of this study can aid in the analysis of safety and mobility issues related to partial lane closures, stopped vehicles and similar obstacles. Keywords: Driver behaviour, obstacle size, on-road static obstacles, lateral gap, vehicle speed. VEHICLES frequently stop on roads for a variety of rea- sons, including mechanical failure, flat tires or police intervention. If shoulders are present, vehicles can pull over without obstructing the roadway. However, in many instances, vehicles stop on road segments without shoul- ders, resulting in full or partial blockage of the carriage- way. Similarly, there are often lane closures following a vehicle crash that may block the carriageway. Since these events are unplanned, there is no way to provide suffi- cient advanced warning to drivers. Obstacles present on the carriageway influence drivers significantly, especially when they are introduced without advanced warning. The behaviour of a driver in the presence of an obstacle on the carriageway also has cascading effects on upstream traffic and can lead to serious consequences, such as dan- gerous driving or traffic bottlenecks. Understanding dri- vers’ actions in the presence of various obstacles is therefore important for both safety and mobility consi- derations. The visual field is the fundamental display for drivers. The drivers gather most information visually. Different objects subtend different visual angles based on their size. Perceived risk depends on the size of the obstacle; the larger the obstacle, higher is the anticipated risk 1,2 . Once drivers detect an object, they determine its hazard potential. Whether an object is sizeable enough to consti- tute a hazard depends on the road width and relative size of the object with other familiar objects 3 . Additionally, the side of the static obstacle can influence the lateral gap maintained from the obstacle. When the obstacle is present on a vehicle’s near side (the driver’s side), the drivers can clearly see the obstacle and estimate its lateral gap. On the other hand, when the obstacle is present on the far side (the passenger’s side), the drivers may not be able to estimate the lateral gap accurately. However, no studies exist that discuss either the influence of different dimensions of obstacle size or the side on which the static obstacle is present. The objective of the present study is to understand the influence of static obstacles on speed and lateral gap, with the side on which the obstacle is present and two dimensions of obstacle size (width and length) serving as the key independent variables. Literature review Driver behaviour is influenced by various factors such as driver experience, and attitude as well as road environ- ment. The different elements of road environment include geometric and infrastructure elements, lane closures, static objects near the road, etc. Influence of road geometric elements (road width and shoulder width) Researchers have studied the influence of narrow roads and road width reduction on speed profiles and lateral placement 4–6 . These studies reported that road width reductions resulted in a significant reduction in vehicle speed. On narrow roads, drivers prefer to drive closer to the road’s centre and avoid overtaking other drivers. Green et al. 7 observed that the standard deviation in