Why Barapullah Flyover Jammed South Delhi Within Just 2 Days: Explained

Construction activity, a college fest and a broken-down bus near Ashram added to Thursday’s congestion. However, police and engineers pointed to the merges as the key problem.

For now, the underlying problem remains the same: a road designed to bring six lanes into three creates a bottleneck where traffic physics, road design and driving behaviour combine to produce congestion.

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Key points generated by AI, verified by newsroom

• New Barapullah flyover opening caused immediate widespread traffic congestion.
• Design flaws include abrupt lane endings and six-into-three lane merges.
• Old planning, driver indiscipline, and traffic physics exacerbate issues.

Delhi’s newest flyover took just two days to trigger widespread congestion across parts of south Delhi.

Barapullah Phase-III, which connects Mayur Vihar to the existing elevated corridor, opened to the public on Wednesday. By Thursday evening, the 8-km stretch between Safdarjung Hospital and Sarai Kale Khan was taking more than an hour to cover, with traffic queues extending across the Ring Road, Outer Ring Road, Mathura Road, the DND flyway and Bhairon Marg.

A Delhi Traffic Police drone survey identified two key design issues. At several locations, two three-lane roads merge into a single three-lane corridor. At another point, where the new spur joins the existing flyover over Sundial Park, one lane ends abruptly.

Construction activity, a college fest and a broken-down bus near Ashram added to Thursday’s congestion. However, police and engineers pointed to the merges as the key problem.

A PWD official told HT that the project's planning studies were more than 20 years old.

The Science Behind Traffic Jams

Traffic flow has been studied as a form of fluid movement for decades. In 1955, British mathematicians James Lighthill and Gerald Whitham proposed treating traffic as a fluid. American engineer Paul Richards independently developed a matching model a year later.

Their model predicts that as cars are added to an empty road, the number of vehicles passing a point each hour initially increases. But after traffic reaches a critical density, drivers slow down to maintain safe gaps.

That makes every journey through the road take longer and eventually reduces the number of vehicles passing through each hour.

Engineers represent this relationship through an inverted U-shaped curve known as the fundamental diagram. The top of the curve represents road capacity — the maximum number of vehicles a road can move in an hour.

Once traffic crosses that point, every additional vehicle reduces the number of cars that can get through.

How A Small Brake Can Create A Massive Queue

The same phenomenon can be seen whenever freely moving traffic encounters a queue.

A vehicle reaches the back of the queue and brakes. The vehicle behind it brakes again, followed by the next one. The braking wave moves backwards through the traffic, even though vehicles at the front of the queue continue moving slowly.

Physicists call this moving boundary a shock wave. In traffic, it represents the transition from sparse, fast-moving vehicles to densely packed, slower traffic.

This helps explain how a bottleneck near Sundial Park can eventually create congestion several kilometres away at Ashram.

Traffic Jams Can Form Without An Obvious Cause

A 2008 experiment by Japanese physicists demonstrated how traffic can become unstable even without a junction, breakdown or road closure.

Researchers placed 22 cars on a single-lane circular track measuring 230 metres and instructed drivers to maintain 30 km/h while keeping even gaps.

Within minutes, small variations in spacing grew until a group of cars stopped. The resulting jam travelled backwards around the track “like a solitary wave”, according to the team in the New Journal of Physics.

“A bottleneck is only a trigger and not the essential origin of a traffic jam,” the researchers concluded.

Drivers themselves contribute to the instability. A slight delay in reacting to a vehicle ahead can lead to progressively harder braking further back in the line.

A road operating close to capacity can therefore develop a traffic jam even without an external trigger. Add a bottleneck, and the chances of a jam increase sharply.

A year later, an MIT-led team described such waves as “jamitons” and showed that their equations matched those of detonation waves in explosives.

Three Design Problems At Barapullah

Lane Balance

The American AASHTO Green Book, a design reference used worldwide, says the number of lanes after a merge should be “not less than the sum of all the lanes on the merging roadways less one”. It also says roads should drop “only one lane at a time”.

That means two three-lane roads should have five lanes after merging. Barapullah, however, has three.

“The merger leads to a deadlock where six lanes try to merge into three,” Dinesh Kumar Gupta, additional commissioner of police (zone-II traffic), told HT.

Abrupt Lane Endings

A lane that ends should narrow over a sufficiently long stretch to allow drivers to find gaps while maintaining speed.

An abrupt lane ending, however, forces vehicles to move across within a short distance.

Researchers Jorge Laval and Carlos Daganzo showed in 2006 that “lane-changing vehicles create voids in traffic streams, and that these voids reduce flow.”

Each forced lane change can therefore generate another backward-moving traffic wave.

“Auxiliary lanes are created here but are poorly designed, which lead to sudden merging of vehicles,” said S Velmurugan, former chief scientist and head of traffic engineering and safety at the government's Central Road Research Institute (CRRI).

Capacity Drop

Once a queue develops, a bottleneck can discharge fewer vehicles than it did immediately before the queue formed. Traffic engineers call this phenomenon a capacity drop.

A congested merge can consequently reinforce the very queue it created.

Delhi’s Driving Habits Add To The Problem

The design principles behind traffic management assume a degree of lane discipline. Delhi’s roads, however, have a mix of cars, buses, two-wheelers and autorickshaws, along with frequent lane changes.

A 2011 paper in Transportation Research Part B by Rahul Nair, Hani Mahmassani and Elise Miller-Hooks modelled such traffic indiscipline as fluid moving through a porous material, similar to water passing through sand.

It found that “drivers of smaller vehicles exploit their maneuverability to move ahead through lateral gaps”. As traffic density rises, those gaps quickly disappear.

Shoulder-cutting can make matters worse. A driver using the hard shoulder may move ahead of several vehicles but then has to force their way back into the queue.

That creates another forced merge, another braking wave and, while the vehicle occupies the edge of the road, one less usable lane.

Germany has legislated a different approach. Section 7(4) of its road traffic regulations requires drivers in an ending lane to use it until the end and then merge alternately, in a zipper-like pattern.

Why A Traffic Signal Could Help

Delhi Police have proposed a signal where Sundial loop traffic meets vehicles coming from Mayur Vihar. If implemented, Barapullah would have two signals, something relatively unusual for an elevated road.

Engineers refer to this approach as ramp metering. The principle is to release vehicles at a rate that the main road can absorb, keeping traffic density below the point at which flow begins to collapse.

Minneapolis-St Paul tested the consequences of removing such controls. Under a state law, around 430 ramp lights were switched off in 2000.

An independent evaluation found that peak-period freeway throughput fell 14%, travel times increased 22%, reliability worsened 91% and crashes rose 26%.

Delhi Police have also asked the PWD to install barricades to create a gentler merge. Gupta said this would reduce the impact without removing the underlying problem.

Sarika Panda Bhatt, co-founder of the Raahgiri Foundation, suggested gradually reducing the number of lanes from three to two and then one.

“This way vehicles do not suddenly see a narrow passage from a wide flyway,” she said.

More Roads Can Also Mean More Traffic

Adding road capacity does not necessarily guarantee less congestion.

Economists Gilles Duranton and Matthew Turner found in 2011 that driving in US cities increases in proportion to new highway lane-kilometres.

Barapullah has already diverted traffic away from NH-24 and the DND.

Mathematician Dietrich Braess demonstrated in 1968 that adding a road can sometimes slow an entire network. Seoul, meanwhile, saw traffic improvements after an elevated highway was demolished to restore the Cheonggyecheon stream.

Navigation apps can further complicate the situation. Without live traffic data for the new stretch, they continue directing vehicles towards it. Delhi Police have therefore written to Google.

What Can Be Fixed — And What Cannot

Barricades, staggered lanes and traffic signals could serve as immediate retrofits. A PWD official said the department was “now discussing what can be done as a retrofitting measure”.

Research has also explored whether automated vehicles can reduce traffic instability. In a 2018 rerun of the circular-track experiment, one self-driving car among around 20 human-driven vehicles reduced braking events by up to 99%.

For Delhi, however, such technology is unlikely to provide an immediate solution.

Experts who spoke to HT said lasting relief would require fewer private vehicles and better public transport.

For now, the underlying problem remains the same: a road designed to bring six lanes into three creates a bottleneck where traffic physics, road design and driving behaviour combine to produce congestion.