Under pressure from the European Union to tackle its air deadly air pollution, the German government is considering making public transport free in its most polluted cities. Few cities have attempted such a scheme — with the notable exception of Tallinn. We asked transport expert Oded Cats, who authored an in-depth study on Tallinn's fare-free scheme, whether Germany can reduce pollution by emulating the Estonian capital.

DW: Why did Tallinn introduce fare-free transport - and was it effective?

Oded Cats: The aims were to promote public transport, to reduce car traffic, and especially to improve mobility for low income and unemployed groups. The latter, I think, we can consider has been achieved. So we do see that a lot of people from low income groups, the unemployed, do travel more frequently.

When it comes to shifting into public transport, we saw an immediate effect, which was fairly small. But about a year or two years after it was introduced, we saw a more long-standing effect: a roughly 14 percent increase in public transport [users] — a large share of which is coming from people who used to walk. So it's questionable whether this is desirable.

If you can take public transport for free you may substitute the short trip you used to walk for public transport. Most of the increase in public transport ridership stems from either people who walked previously, or previous transport users who travel more frequently or perform longer trips. Only a small part of those additional trips come from people who also used the car. So we cannot say that there was a net gain in terms of reducing car traffic, or the congestion and emissions associated with it.

Why didn't the scheme attract more drivers?

We know from many other places where people have experimented with short-term campaigns of free public transport that price is seldom the reason that you choose car over public transport. We see that choices of which mode to use are much more to do with service qualities. So for example, if this investment is made in increasing frequency rather than reducing the prices you're much more likely to gain new customers.

So what should Germany be doing to get drivers to make the switch?

Not only from Tallinn, but also from a lot of other places we have a large amount of evidence that what is most effective is to increase the price of using your car — and specifically things that relate to using it per trip, like fuel and congestion taxing — as well as parking. Things that are really attached to you using your car, not owning one. Making sure that drivers pay for ­what we call the externalities — undesired effects — that car traffic causes, namely air pollution and congestion. And this you do via parking charges, via congestion charging, fuel taxes. And only when drivers encounter the real cost of the choice they make, they can make a more informed decision. And you do this while having a very competitive alternative in the form of public transport.

What can we learn from other free public transport schemes around the world?

Most of these schemes are taking place in Europe, and also mostly in small towns, which are quite particular user groups. Some of these towns report tremendous increases in the number of riders. When you look closely into the data, you see that these numbers are very small and there was also a simultaneous increase in the supply offered. So instead of having two buses a day you have ten buses, and then your report a 100-percent increase. So we don't see evidence of a large effect from this.

There are several cities, the most well-known of which is Hasselt in Belgium, which had it for many years and had to change back to charging users because it was not financially sustainable. They found it quite a painful choice to make to reintroduce charges to make the system viable again.

How did Tallinn — a city of half a million people — cope with the cost?

In Estonia local taxes are attached to the place of residence. And then there was a sizable group of people living in Tallinn that was still registered as residing in other towns in Estonia, and this was a lost tax income for the city. This was a way also to appeal to these people to register in Tallinn because you're eligible for people transport entirely but only if you're registered as a resident. So effectively, by attracting this additional tax income they could cover the costs of the lost income from public transport tickets.

Oded Cats is assistant professor of transport and planning at Delft University of Technology in The Netherlands, and part-time researcher at the Royal Institute of Technology, Sweden. His research focuses on the intersection of transport networks, operations, policy and travel behavior. He is also associate editor of the European Journal of Transport and Infrastructure Research (EJTIR), and a member of the editorial board of Journal of Public Transportation and Journal of Urban Rail Transit.

This interview has been edited for clarity.