The Mumbai-Pune Expressway is a high-speed six lane corridor accessible to passenger cars, trucks, and passenger taxis. The speed limits of the expressway range between 80 and 120 kilometres per hour. A part of the expressway is in the Ghat region and there are several horizontal curves. So, the speed limits are reduced to range between 80 and 100 kilometres per hour. Yet many accidents take place there. In fact, last year it was referred to as a death trap.
Image: Neelnimavat via Wikimedia Commons
Researchers at the Sardar Vallabhbhai National Institute of Technology, Surat have been studying road geometry and vehicle performance on the horizontal curves of hilly terrains. Recently they evaluated and reassessed the design of the 22 curves in the ghat section.
Curve geometry, they noted in earlier studies [1, 2 ], influences the driver’s decisions about speed when navigating horizontal curves. Besides speed, the main factors that lead to accidents are the distance the drivers can see when taking a curve, and superelevation, the differences in the height of the inner and outer radius, which provides a slant to the road to offset the centrifugal forces on vehicles taking the curve.
All three parameters are complex and subject to non-compliance. Drivers may disregard speed limits. The tyres of the vehicles may be worn out. The road may not have adequate friction, complicated by rain or loose gravel leading to skidding, which leads to accidents even if there is adequate superelevation. The distance visible around the curve may not be in tune with the requirements and may not give adequate time to decelerate or stop to avoid accidents.
To examine the issue of accidents in the Mumbai Pune Expressway, the researchers collected the geometric data of the expressway from the Maharashtra State Road Corporation. The design of the road, they found, was as per the the Indian Road Congress design guidelines. The minimum curve radius of the 22 curves followed the recommendations: 260 metres for 80 kilometres per hour, 440 metres for 100 kilometres per hour and 670 metres for 120 kilometres per hour. There were minimal variations for the other parameters, such as superelevation and the grade of the road, which determines the friction necessary to stop the vehicle. So the researchers focused on the speed and the distance needed to stop the vehicle at the given speed.
They used three radar guns to collect data on speed – one at the entry point to each curve, one at the middle of the curve, and one at the exit point of the curve. While one enumerator used a radar gun to record the vehicle’s speed, the other noted the license plate. Thus, six enumerators, two at a time at each location, gathered the vehicle speed data of the same vehicle at three locations on each curve.
Data was collected for three vehicle types: trucks, passenger cars and passenger taxis, going in both directions. When the researchers analysed the data, they found that private cars were more non-compliant about speeds than passenger taxis. The mean speed of trucks was even lower than the mean speed of the taxis. Truck drivers had greater attention to detail and compliance with regulations in the expressway. This may be because of higher loads, the risk of toppling and the fear of consequent financial losses.
As the curve radius increased and, consequently, the visible distance around the curve also increased, this tendency of passenger cars to overspeed increased. So, the safety margin for passenger cars was reduced.
As the curve geometry changes, the standard deviation in speed also changes. Drivers of vehicles in the inner part of the curve and those on the outer part of the curve perceive different geometries and have different visible distances. However, this is not taken into account in the design of the road in curves.
The system reliability for non-compliant cars was negative, especially for curves 8 to 10 and 19 to 21 which had a lower curve radius.
The speed stopping distance, which depends on curve radius, sight distance, perception to reaction time, friction and vehicle speed, is a better indicator of system reliability than speed or curve radius alone, say the researchers.
India accounts for 11% of the world’s road accidents. Over 450,000 accidents were reported in 2022 with over 150,000 deaths as per the Ministry of Road Transport and Highways [3]. What needs to be done to reduce fatality and morbidity due to road accidents?
Transportation Research Record 1–13;
DOI: 10.1177/03611981241310128
References:
[1] DOI: 10.1007/s40890-025-00241-5
[2] DOI: 10.1061/JTEPBS.TEENG-7919
[3] Road Accidents in India, 2022 (Ministry of Road Transport and Highways
Reported by Atig and Udham
Freelance writers, Goa
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