PHILADELPHIA—Traffic crawls along downtown’s Chestnut Street, horns honking as cars block the intersection. Up ahead, construction claims one of three lanes, and a Canada Dry delivery truck is parked in the bus lane, forcing city buses to squeeze by.

Similar scenes play out every day in cities across America. In six of the 10 most populous cities, including Philadelphia, average midday vehicle speeds in the downtown core have slowed between 3.8% and 10% since 2015, according to an analysis for The Wall Street Journal by transportation analytics firm INRIX. Speeds have decreased the most in Phoenix.

“We are at a point on many streets of just incredibly frustrating gridlock,” said Paul Levy, chief executive of Philadelphia’s Center City business improvement district, which encompasses 233 blocks. Although the snarls reflect vitality, he said, “it is clearly a situation which can’t be allowed to continue.”

The problem has many causes: a surge in ride-hailing services such as Uber; a burgeoning downtown population; construction; the creation of more bike lanes; and a rise in deliveries fueled by e-commerce, city officials said. The combination is clogging a grid of narrow streets laid out in colonial times.

The U.S. is the most congested developed country in the world, according to INRIX. American drivers spend an average of 41 hours a year in traffic at peak commuting hours, its analysis found, collectively costing drivers about $305 billion in 2017.