In statements issued before the official release of the report, the companies pushed back against the notion they're responsible for worsening traffic conditions. Uber said the study failed to account for a dramatic increase in tourism in recent years or traffic bottlenecks caused by more frequent freight and e-commerce deliveries in the city. Lyft pointed to studies that suggest TNCs may actually reduce congestion.

Findings Add to 2017 Report

It's not exactly news that there are lots of Lyft and Uber vehicles on San Francisco streets. If you're in the Financial District, South of Market (SOMA) or the Mission and open the companies' apps, nearby thoroughfares appear to be crawling with ride-service cars.

That impression is backed up by data. The SFCTA said in an analysis of TNC traffic last year that as many as 6,000 Uber and Lyft vehicles were on the city's streets at any one time -- dwarfing the city's mostly moribund taxi fleet -- and accounted for about 170,000 vehicle trips on a typical weekday. That ride service traffic -- again based on data from late 2016 -- represented about 15 percent of all vehicle trips in the city, the SFCTA said.

"When we put that report out, what it revealed was that there are a lot of TNC trips happening, oftentimes in the most congested parts of the city and often at the most congested times of day," said Joe Castiglione, the SFCTA's deputy director for technology, data and analysis.

"However, we couldn't really say how much TNCs were contributing to congestion or how they might have changed congestion since they came on the scene around seven or eight years ago, because lots of other things have changed in the interim," he said.

Among those changes: the city's spike in employment and the increase in its population.

"Certainly all of these new people -- new residents and jobs -- are also contributing to congestion," Castiglione said. "What this new report does is answer the question, 'How do TNCs affect roadway congestion in San Francisco relative to all of these other factors?' "

The SFCTA report acknowledges a host of unknowns in answering that question, noting a lack of data on how delivery services, freight traffic and construction activity on city streets might be affecting congestion.

The study also concedes that it's not a given that ride services increase congestion. For instance, Uber and Lyft could encourage transit ridership by serving as a bridge to and from train stations and bus stops.

S.F. Streets: More Delays, Lower Speeds

But after sifting through data from several sources, the transportation researchers concluded that the ride services are, indeed, a major factor in traffic congestion.

Using information on late 2016 Uber and Lyft activity in San Francisco, obtained by researchers at Northeastern University and data about citywide traffic conditions from transportation intelligence firm INRIX, the SFCTA analyzed three measures of congestion: the extent of travel delays, the total number of miles vehicles travel in the city every day and average traffic speeds.

The SFCTA says that total daily traffic delays in San Francisco increased from about 65,000 hours in 2010 to 105,000 hours by the end of 2016. By analyzing several different scenarios, including a hypothetical situation in which there was no TNC traffic in 2016, the agency estimates that 51 percent of the additional 40,000 hours of daily delay was due to the ride services.

Similarly, the study found that the TNCs contributed heavily to increases in the number of vehicle miles traveled each weekday and decreases in average speeds on city thoroughfares.

The SFCTA report says weekday vehicle miles traveled increased from 4.9 million to 5.6 million between 2010 and the end of 2016, with TNCs accounting for 47 percent of the 700,000-mile increase. The city researchers estimate that average speeds on city streets fell from 24 mph to 20.9 mph during the study period, with ride-service traffic responsible for 55 percent of the 3.1 mph drop.

Hardest-Hit Neighborhoods

The report also highlights the neighborhoods and times of day where the ride-service firms have had the biggest impact on travel conditions.

Among those findings:

Ride-hailing companies are major contributors to the horrendous traffic conditions in the city's Financial District and SOMA, areas which have seen major increases in travel delays and the number of vehicle miles traveled.

The Mission District saw the biggest drop in average traffic speeds -- almost 5 mph over the six-year study period. Smaller but significant drops were also seen in the Financial District, SOMA and the Civic Center/Hayes Valley/Panhandle corridor. The study says ride services were the biggest factor in the speed decreases in those areas.

One SOMA block offers a dramatic example of both worsening congestion and the role ride-service firms play. The SFCTA says daily hours of delay more than doubled on Bryant Street between 5th and 6th streets-- a key approach to the eastbound Bay Bridge. The agency says 80 percent of that additional congestion is due to ride-service vehicles.

Traffic delays have increased and speeds have slowed at all times of day. The report says the TNCs have had the most dramatic relative impact on evening and overnight traffic, when average traffic speeds citywide have dropped from about 26 mph to about 22 mph.

The data confirm that Valencia Street, often the scene of evening TNC scrums, has been severely affected by the advent of Lyft and Uber. Evening congestion has skyrocketed on the popular Mission District strip, virtually all due to ride-service traffic. Lyft has tried to address that issue by ending passenger pickups on Valencia between 16th and 19th streets, directing customers to catch their rides on side streets.

Castiglione and other SFCTA researchers who worked on the study are scheduled to present their findings to the Board of Supervisors -- meeting as the county Transportation Authority Board-- on Tuesday.

Uber and Lyft criticized the agency report, saying the findings don't fully account for other sources of traffic congestion and fail to credit ways in which ride services may ease travel overall.

"While we appreciate efforts to better understand the causes of congestion, this study fails to consider critical factors like the spike in tourism or the growth of freight deliveries, both of which have exploded since the study’s baseline date of 2010," Uber said in a statement.

Lyft representative Lauren Alexander said via email that the SFCTA report "is flawed and an incomplete picture of the transit challenges San Francisco faces. Congestion is a complex issue, and Lyft is committed to being a part of the solution. Lyft stands ready to partner with SFCTA to advocate for solutions like congestion pricing and building more housing near transit to address the core causes of congestion."