Why is traffic so slow in some cities? What can be done to improve traffic speed and reduce congestion? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world.

Being stuck in slow traffic is one of the most frustrating daily experiences for many people. So it’s a great question to ask: why is traffic so slow in some cities, yet so fast in others?

My research Akbar et al. (2023) used Google Maps data to rank the world's slowest and fastest cities, revealing insights into the causes of slow traffic speeds. It turns out that the smaller U.S. cities like Flint, MI, and Greensboro, NC, have the fastest road traffic. On the other hand, larger and less affluent cities like Dhaka (Bangladesh), Lagos (Nigeria), and Manila (Philippines) have the slowest road traffic. Here is a figure comparing traffic speed throughout the day in various cities:

Figure note: Mean speed for trips with lengths between 5 and 10 kilometres. Dhaka is the slowest, Flint is the fastest, and Bogota is the most congested city in the world. Central Dhaka refers to trips that take place on average within 5 kilometers of the center of Dhaka.

In general, cities with the fastest traffic share two broad features: they have small populations, and they are located in rich countries. Conversely, cities with the slowest traffic are very large and very poor. You may be wondering how population size and income levels affect traffic speed. Let me tackle these two factors separately.

A) Why are more populous cities slower?

Large cities are denser and more congested. However, contrary to popular belief, congestion isn’t the main issue. Even at 3am with fewer cars on the road, megacities experience slow traffic. A large and dense population implies that many other road users are competing for road space with vehicles. These competing road uses might include encroachment by double-parked delivery vehicles, electric scooters, draft animals and pedestrians, limited visibility due to buildings close to roadways at intersections, and longer waits at intersections where red lights replace stop signs and roundabouts.

B) Why do poor cities experience slower traffic?

The primary reason is the lack of adequate road infrastructure like major arteries and other fast road types. Additionally, cities in poorer countries tend to have higher population densities. This increased density leads, as explained above, to more competition for road space by other road use in poor countries.

There are also a number of other factors which, while less critical, still affect traffic speed. For example, a city's regular grid network can significantly improve traffic flow. A challenging topography –such as Dhaka being criss-crossed with water bodies – impedes traffic speed.

So, what does this tell us about improving traffic speed? Recognizing that traffic slowness isn't mostly due to congestion has important policy implications. For instance, conventional solutions like congestion pricing and HOV lanes may not significantly improve speed in the slowest cities, despite the desirability of such policies in many cases. Though much more expensive, enhancing road infrastructure would be a more effective solution.

While cities with faster traffic are in rich countries and have smaller populations, there is hope for slower cities. As poor cities grow wealthier, they tend to build faster roads and expand spatially, reducing density, which leads to improved traffic speeds. Achieving major speed improvements in affluent, densely populated cities like New York is a more formidable task. The advantages of such cities, however, is the quick access to many nearby destinations, which compensates for the slower traffic.

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