Drivers for companies such as Uber and Lyft make about 170,000 trips in San Francisco on a typical weekday, accounting for about 15 percent of all car trips made in the city.

That data — part of a presentation that the San Francisco County Transportation Authority delivered Monday to the Board of Supervisors’ Land Use and Transportation Committee — also showed that these drivers log about 1.4 miles for every mile they drive a passenger.

That could partly explain why congestion has gotten so bad in downtown San Francisco, where many of these intracity trips take place. At peak times, speeds on the city’s main thoroughfares hover at just 12 miles per hour, Transportation Authority officials said.

But, they cautioned, the city’s development pattern is also to blame. In the past few years San Francisco has proportionally added more jobs than housing, drawing commuters from all corners of the Bay Area and from as far away as the San Joaquin Valley. While many of those people take public transportation, some drive.

— Rachel Swan

Email: cityinsider@sfchronicle.com, rswan@sfchronicle.com Twitter: @sfcityinsider @rachelswan