SOME cities are so shaped that traffic jams seem inevitable. Manila, in the Philippines, is scrunched between a bay in the west and a lagoon in the east. A car driving through the middle of the city is like a grain of sand seeping slowly past the neck of an hourglass. Dakar, in Senegal, is surrounded on three sides by the Atlantic Ocean, forcing suburban commuters to crawl along the same east-west roads. Auckland, Monrovia, New York and Rio de Janeiro are similarly pinched for space to grow.

Cities can build around natural pinch points by erecting long road bridges, as Lagos and Mumbai have done, or by tunnelling. But that is difficult and expensive. So some governments have a different idea: build somewhere else. Auckland is mulling a new city to the south that could contain half a million people. Liberians have thought about starting again near Monrovia, or moving the capital somewhere else—though Liberia is for now too poor to manage it.

Thirty kilometres east of central Dakar, a huge billboard advertises a forthcoming metropolis, to be called Diamniadio Lake City. Renderings by the developer, Semer Group, suggest Dubai (where Semer Group is based) with added curves and African flourishes. In short, it will be rather like the fictional capital of Wakanda in the film “Black Panther”. There is not much to see yet, besides a conference centre, a hotel and lots of scrub. But the government hopes that 300,000 people will live there eventually, easing pressure on the existing city.

“We pray that it works,” says Ibrahima Ndiaye, an urban planner in Dakar. But he doubts that a new city will be enough to unclog the old one. Senegal also needs to deter people from driving into Dakar. It tried charging for parking in the busiest district, but stopped after complaints, says Mr Ndiaye. This is not just a poor country failing. Auckland and New York have both pondered congestion charges for over a decade; neither has them yet. Many city dwellers seem to prefer traffic jams to parking meters or tolls.