In the coming week, Minister for Transport Eamon Ryan is expected to bring to cabinet a memo containing a suite of measures he says will help to reduce transport-related carbon emissions.

Controversially, among the proposals is likely to be a plan for congestion charges in Irish cities at some point in the future.

The proposal faced staunch push-back from members of Fine Gael and Fianna Fáil, including An Taoiseach Leo Varadkar.

As part of The Conversation from RTÉ's Upfront with Katie Hannon, we asked two people to join our WhatsApp group, to debate the merits of introducing congestion charges in Ireland.

Emer Higgins is a Fine Gael TD for Dublin Mid-West. She opposes the introduction of congestion charges.

Andrew Smith is a researcher who works on the Energy Policy Modelling Group at University College Cork. He is a transport planner who previously worked on the London congestion charge scheme.

Here is their discussion.

Emer Higgins: Hi Andrew, in terms of congestion charges, my view is that I don't think now is the right time to introduce them because I believe our public transport system needs to be developed further first.

BusConnects is being rolled out in Dublin and there are finances in place to build three flagship transport capital projects - new LUAS lines, Metro and an upgrade of train lines to Dart lines.

Let's get these up and running first to give people viable alternatives to driving into our cities before we consider congestion charges.

Andrew Smith: Hi Deputy Higgins, a pleasure to meet you, if only via WhatsApp.

There's a solid body of research, and decades of evidence that offering incentives without disincentives only helps a bit. It's poor value for money, compared to combining incentives for the things we want more of, with disincentives for the things we want less of.

I absolutely agree with you that now is the time to roll out improvements to public transport, as well as better facilities for walking and cycling.

And the best way to do those things, and get outcomes that improve the city for everyone, is to make them part of a coherent set of complementary measures as part of a congestion charging scheme.

London implemented its scheme with all the complementary measures in less than three years. So, we could have this up and running by 2025. Wouldn't that be great?

Emer Higgins: Yes, totally agree with you on the public transport side.

If congestion charges were introduced today, I think they would just add to the cost of going to work, college, hospital or wherever and possibly impact trade and put jobs at risk.

I'm not sure they'd even achieve their objective because without providing viable alternatives to driving I can’t see car dependency reducing.

I know that the success of congestion charges in London is well documented. But London has 11 Tube lines and 272 stations on top of their bus network and trains. We don't have anything comparable.

I live in Lucan so I'm on the new C Spine bus route and it's brilliant, but there's still parts of Lucan that are under-served.

When we've more buses, our train line to Adamstown electrified to DART+ and a Luas to Lucan then we'll be in a totally different space.

Andrew Smith: I agree that we couldn't have a successful congestion charging scheme without improving the alternatives to the car.

We can't make more space for buses without taking cars off the road. And we can't cost-effectively meet our carbon budgets without taking cars off the road. So, the key question is: what is our mechanism for doing so?

Congestion charging is only one possible answer, but it's a powerful one. Congestion is a cost in itself. It's a cost to every car, every bus, every ambulance and every business using the city streets. When we price congestion, we improve life for all of those people. And those people who do keep driving, are paying more, and they're getting a faster more reliable journey as a result.

It's not the only way to do it. Instead, we could ban any car that wasn't zero emissions from entering the city centre.

That would be effective for a while. If we want to meet our legal and moral obligations, we have to find a disincentive to car use to get value for money from all the incentives for walking, cycling and public transport.

Emer Higgins: I agree that the cost of congestion itself has to be addressed.

I do think banning non-zero emission cars would be difficult to police and also regressive as it tends to be higher income earners who can afford zero emissions cars.

Putting a tax on everyone else wouldn't be fair without a ramping up of EV grants and charging facilities which is ongoing to be fair but not at the stage where we could ban all cars other than zero emissions.

I know the only way to achieve less congestion is to have less cars on the road but until we've viable alternatives to get around I wouldn’t be in favour of charges.

It needs to be easier, cheaper and safer to leave the car at home. We have plans in place to deliver these multibillion-euro projects. We need to ramp up investment and get them done asap so we can encourage more people onto public transport rather than punishing them out of their cars.

Andrew Smith: It's not punishing though. We accept that we ration using pricing on a vast number of things. The alternative is a Soviet-style queueing system. If we can agree that markets can work, then we can agree on value of pricing scarcity.

It's good economics to put a price on activities where taking part in that activity, makes things worse for other people. Driving in the city is one of those activities.

You're absolutely right on the issue of enforcement being important. Doing any of the things we need to do will require a step up in enforcement compared to what we're used to. That's true of illegal parking, of misuse of yellow box junctions and bus lanes.

As the recently published National Transport Authority work showed, even making public transport free would only give a 1% reduction in car use. There's a lot of experience around the world in cutting car traffic, and doing it fairly and well. There is another way. And that's to close off much of Dublin city centre and Cork city centre to private cars completely. That's something that could be done pretty quickly and Paris has been hugely successful at it.

Emer Higgins: There is already many costs to driving – fuel for one. And that price is taxed too. Some would say that that is a disincentive, especially at the moment. Yes, we will need more disincentives but only when there's enough options other than driving on offer.

Let's say we started taxing people to drive into the city tomorrow – there'd be a portion of people who could afford to pay the charge and would continue to do so and then the people who couldn't afford the charge would have to turn to public transport and we'd have over crowded carriages and full buses whizzing by packed bus stops. Regular commuters would be completely disrupted and the public transport system wouldn't cope.

My worry would be that we'd end up with public transport drivers leaving and that would exacerbate the driver shortage we already have. Some of the biggest barriers for people choosing public transport are reliability issues and lack of access to routes and those two problems are driven majorly by a lack of drivers. If you live in Newcastle in Dublin for example you know all about unreliable buses on the 68 route.

If it's a decision between that scenario or incremental progress in line with the roll out of more public transport services, then that would be my choice.

I suppose I’m always thinking with constituents in mind, and I know that emission reductions and climate mitigation measures are a priority for them but a lot of them in more rural areas of west Dublin are badly served by their current bus routes.

I agree we need to reduce congestion and reduce emissions, and yes congestion charges would help make that happen. But unless it's coupled with viable alternatives to driving, I think it would be too punitive. When we've the LUAS and Metro up and running then is the time to look at options you've raised around stronger disincentives.

That's my view, but I do respect and mostly agree with what you're saying.

Andrew Smith: I'm very glad that I merely have the simple job of sifting through the evidence and advising policy-makers on what does and doesn't work.

I don't envy your task at all. You have the difficult job of having that honest conversation with the voters about the difficult trade-offs that have to be made and working out how we make the less comfortable essential changes that actively deter some car use, because that's how we will get faster, more frequent and more reliable buses. And better streets for people and for business.

If we don't do congestion charging (not going live tomorrow, but starting developing it tomorrow, so that it can go live in 2025), we'll need to pick another disincentive that makes all the incentives work properly. The only way we fail, is if we pretend that we can get better streets without using disincentives to the car.

I hope this conversation is the start of a long and fruitful journey that will achieve the outcomes we all want.

Emer Higgins: I hope so too. Thanks for engaging with me on this Andrew. Different perspectives and diversity of thought need to be around every decision-making table!

Andrew Smith: Thank you too, Emer.