EW: Well, the first thing is the amount of time those cars are traveling without a passenger. So, when you drive your own car from one place to another, that's just Point A to Point B. Lyft is going to take you from Point A to Point B and then is going to drive around waiting for someone else to join. So that looks like something that could add to miles traveled on the road.

OAP: Are there more cars on the road because of services like Uber and Lyft?

EW: Well, San Francisco was never a major taxi town. Even in 2012, before these services got big, taxis made up only around 1 percent of rides within San Francisco. Now a study by the county of San Francisco estimates 15 percent of trips inside the city are made through Uber and Lyft. That's huge. That's much bigger than taxis used to be.

OAP: How many Uber and Lyft cars are actually on our streets?

EW: According to data gathered by the county of San Francisco, on an average weekday there are more than 5,700 Uber or Lyft vehicles in San Francisco during peak hours. On Friday, that goes up to 6,500.

OAP: What kind of rides are Lyft and Uber replacing? Are people using public transit or their personal cars less?

EW: The impact on transit is pretty complicated. It seems from a couple of studies, if you're talking about a shorter trip, people are now more likely to choose not to take the bus and choose to take an Uber or Lyft instead. For a longer trip, it may be that some people are choosing to take public transit more and take Uber and Lyft towards the end of the trip.

BART actually did a study of their own riders who use Uber and Lyft, and about the same number said it made them more likely to ride BART as the number who said it made them less likely to ride BART. But I think a more key thing to look at is whether Uber and Lyft are creating rides where before someone wouldn't have done a ride at all. And we don't have data on that for San Francisco. But a study based out of UC Davis found that 49 to 61 percent of ride-hailing trips would not have been made by a car at all.

OAP: Wow. So that would be a lot more cars on the road.

EW: That would be a lot more cars.

OAP: And is there a scenario in which Uber and Lyft actually help reduce traffic? Because I imagine you could get to a point where so many people are relying on these services that they wouldn't have cars of their own, and that could be a good thing.

EW: Yes, it's possible. You can imagine ways that they would reduce traffic. If people are circling for parking less and taking those services more, and they're so efficient that they're always picking somebody up, that could reduce traffic. If people are taking public transit more and using Uber and Lyft for just the last bit of the journey, that could reduce traffic. The biggest way they could potentially reduce traffic is if people start to use it more as a carpooling service. So both Uber and Lyft have a carpool version of the service. For now they're not as popular, but both Uber and Lyft are saying they want to increase the number of rides that happen through carpooling. … But you still have to remember, if you take three people out of a bus and put them into an Uber, that's still increasing the number of vehicles on the road.

OAP: And what can governments do? Can our cities actually help with this problem?

EW: A lot of transportation planners say if you really want to decrease traffic, the way to do that is to charge people for driving. So right now there's some talk in San Francisco of imposing some sort of charge for cars entering the city. Another way to do that could be to charge for every mile that people drive and you could put that kind of tax specifically on Uber or Lyft. Right now individual cities don't have the authority to do that. In California, authority to regulate these companies is held by the state, and they're not making moves like that. They're not under the same kind of pressure of traffic as San Francisco.

OAP: So is there any hope for our listeners who just want some traffic relief? Is there anything that they can look forward to?

Well, I hope they have lots of things they can look forward to, but for traffic relief they’re going to have to hold out for a world where people are carpooling more. There is a hopeful case for when driverless cars come along, our whole network is going to be so smart that people are going to be carpooling all the time and there aren't going to be more cars on the road than there have to be. I'm worried that the reality is going to be the opposite. As driverless cars come in, it costs even less to have a car out on the road than it does now. I wish I had good news for listeners, but I don't feel like I do.