While traffic is expected in major metropolitan locations like New York City, a recent report reveals that the issue is worsening in there -- with traffic slowing by 20 percent -- due in part to the growth in popularity of ride-hailing apps like Uber and Lyft.

Looking at data over a four-year period, the research, conducted by transport expert Bruce Schaller, revealed that the number of cars in Manhattan associated with ride-hailing services like Uber and Lyft grew by 81 percent. In fact, the roughly 68,000 vehicles associated with those companies significantly outnumber the traditional yellow cabs licensed to operate in the city by almost five-fold.

Additionally, the researchers also estimate that Lyft- and Uber-associated drivers spend at least 45 percent of their time just driving around looking for fares, subsequently clogging the busy streets with empty cars.

“Lots and lots of people are deciding that, 'Oh, public transport is just too much of a hassle this morning,' or whenever, which causes a shift in patronage from public transport to ride-sharing services," said Jarrett Walker, a public transport policy expert. "That means moving people from larger vehicles into smaller ones, which means more vehicles to move the same people. Therefore, more traffic."

A computer scientist at Northeastern University agrees that the ride-sharing apps add to the issue of congestion.

"You can look at the traffic pattern for the Uber vehicles and it perfectly matches the peaks for the rush hour and the peak time of day," said Professor Christo Wilson. "They are out there in force at the worst possible times."

Likewise, Wilson believes that the apps are also contributing to other traffic-causing trends.

"It is increasing the total number of trips and these are discretionary trips that these people would not have taken if not for cheap, available ride-sharing," said Professor Wilson. "So, that is almost certainly increasing congestion."

However, ride-sharing firms deny that they are the primary cause of congested city streets and believe instead that other factors are at play, including road construction, economic growth and faulty policies designed to streamline traffic.

"The number of drivers we have on the road is one of the easiest things to measure and that often becomes the focal point of the conversation," said Andrew Alzberg, Ubers’ head of transport policy.

For more on the study, click here.