Kolkata’s painted flyovers face scrutiny after videos of bikers slipping on the surfaces went viral, prompting a state government inquiry.
Asphalt gets its black colour from bitumen, while road safety depends on surface texture and sufficient tyre friction.
Authorities have scraped some paint and installed reflectors as TMC MLA Kunal Ghosh questioned the decision to paint the flyovers.
Kolkata’s brightly coloured flyovers, introduced as part of a Durga Puja beautification drive, have come under scrutiny after videos of bikers skidding on the painted surfaces circulated widely on social media. The West Bengal government has ordered an inquiry to determine whether the material used on the roads contributed to the incidents.
The Opposition Congress and the Trinamool Congress (TMC) have criticised the BJP government, alleging that the paint made the roads unsafe. Urban Development Minister Agnimitra Paul, however, said anti-skid paint had been used for the project.
The controversy has brought attention to a basic question of road engineering: why are most roads black, and what determines whether a road surface is safe for motorists?
What Is Happening In Kolkata?
The state government’s inquiry will examine the material applied to the flyover and whether it played a part in the skidding incidents.
“Officials have been directed to investigate the cause of the accident. Some people are saying that the colours are straining the eyes while driving. If that is the case, an alternative arrangement will be made. Whatever is convenient for the people’s needs will be done,” Paul told mediapersons.
On Wednesday, October 15, authorities installed reflectors on the coloured roads and scraped off some of the paint. Paul said cat’s-eye reflectors had been installed on Chingrighata bridge, while paint on the Gariahat flyover had been scraped slightly to increase friction.
The Chingrighata flyover remained closed through Wednesday night as repair work and reflector installation continued.
On Thursday, TMC MLA Kunal Ghosh, from the Mamata Banerjee faction, shared a Facebook video showing paint being washed away from the Gariahat flyover. Questioning the decision to apply it, he asked who would be held responsible for the accidents and the expenditure.
The inquiry is expected to establish whether the treatment used on the flyovers affected road grip and what corrective measures are required.
The Science Behind Black Roads
The familiar black surface of most roads comes from asphalt, a construction material made by combining mineral aggregates, including crushed stone, gravel and sand, with bitumen. The latter, a dark binder derived from petroleum, holds the aggregates together.
The mixture is laid while hot and hardens as it cools, forming a surface designed to withstand traffic. Asphalt is widely used because it is strong yet flexible, and roads made from it can be constructed and repaired relatively quickly.
Concrete is another material used in road construction. It generally costs more and takes longer to build with, but concrete roads can carry heavy loads better and last longer.
The colour of a road also has practical implications. Black surfaces absorb light, whereas bright colours may reflect more light and cause glare for drivers. Lane markings are also easier to distinguish against a dark background.
Colour, however, is only one consideration. The ability of a road to provide adequate grip is central to its safety.
Why Grip Matters
Tyres rely on friction with the road to brake, accelerate and change direction. When that friction is inadequate, vehicles can skid, particularly when the surface is wet.
The US Federal Highway Administration (FHWA), in its technical advisory on pavement surface texture, identifies microtexture and macrotexture as essential to maintaining friction in wet conditions, reported Indian Express.
Microtexture refers to the small irregularities on the surface of the stones used in a road. These help tyres maintain contact and grip. Macrotexture describes the larger variations in the road surface, which help water drain from beneath tyres, especially at higher speeds.
A surface that appears rough is not necessarily safe. Traffic can polish the stones over time, reducing grip even when the road does not look smooth. Equally, a properly engineered surface can provide sufficient friction without appearing coarse.
According to the FHWA, both asphalt and concrete can provide safe, durable and relatively quiet road surfaces when designed and constructed properly. The material used is therefore not the sole determinant of safety; the finish and texture also matter.
Can Roads Be Coloured Without Compromising Safety?
Colour can be incorporated into road construction rather than applied as a separate coating over an existing surface. Construction materials company Holcim explains that pigments can be mixed with hot asphalt binder and aggregates during production. Coloured aggregates can also be used to achieve different shades. The mixture is then laid as the road surface.
Coloured asphalt can be used to distinguish cycle lanes, pedestrian areas and other designated spaces. But a surface intended for pedestrians or residential areas presents different engineering requirements from a busy carriageway carrying vehicles at speed.
