Congestion peaked in the late afternoon in 19 of the most populous 20 cities studied by Geotab with the exception of Baltimore, which had the most gridlocked congestion between 8 and 9 a.m., the telematics provider found.

The speed of Geotab-equipped vehicles fell 40% in Boston between 4 and 5 p.m. and 38% in New York City between 5 and 6 p.m. Washington, D.C. has the slowest traffic with vehicles averaging 19 mph between 5 and 6 p.m. and peaking at 33 mph between 4 and 5 a.m., according to Geotab.

Geotab compiled its results in a series of interactive maps that show congestion by an hourly breakdown for each city. The maps were created using 7.7 million aggregated data records collected over the course of a year by Geotab Go devices.

"Most drivers know when and where they’re likely to get caught in traffic in their area, but these maps allow users to explore the true scale of congestion across America,” says Mike Branch, Geotab’s vice president of data and analytics.

Other cities that experienced at least a 30% drop in speed during the peak traffic hour include Miami (37%), San Francisco (34%), Chicago (33%), Los Angeles (32%), and Seattle (31%).

Cities with the lowest speed drop include St. Louis (16%), Dallas (20%), Phoenix (21%), Detroit (21%), and Baltimore (22%). Vehicles in St. Louis maintaned an average speed of 41 mph between 4 and 5 p.m.

Geotab gathered the data between Aug. 1, 2017 and July 31, 2018.