AS ANYONE WHO has travelled along the quays of the river Liffey in Dublin recently will know, the city centre of our capital city is choking in traffic. The result of this is huge amounts of time wasted by people sitting in buses and cars stuck in congestion.

To address this, Dublin City Council has proposed limiting certain roads in the heart of the city centre to public transport and cyclists from 7am to 7pm.

A group of car park owners and retailers opposing these measures have commissioned an economic analysis by consultants claiming the changes would reduce retail spending in the city centre by €141 million in 2028, leading to almost 1,800 job losses.

However, this negative assessment is based on a deeply flawed methodology, underpinned by dubious assumptions that don’t stand up to the most simple scrutiny.

Firstly, the analysis assumes that 65% of all traffic – car, bike and public transport – that crosses the canal between the hours of 7am and 10am does so primarily for the purposes of retail shopping.

This dubious assumption is made by the consultants using a figure from a survey of people on Grafton Street and Henry Street – two of the main shopping areas in Dublin – carried out in the summer months across the full week including weekends. Applying this 65% figure to the number of individuals entering the city centre during peak commuting hours is clearly nonsense.

Second, the analysis doesn’t directly model or estimate the impact of the measures on transport volumes or shares, or even claim that the measures will reduce car traffic.

Rather, it bizarrely asserts the effect of achieving the transport plan’s goals would be to shift people away from travelling to the city centre by public transport and foot, and towards bicycle.

It then assumes that those who shift from travelling to the city by bicycle will spend between and third and a half less than they otherwise would if they travelled by public transport.

The source for this dubious assumption is the same survey as above, which estimates that cyclists spend less in the city on average than those travelling by public transport.

While the estimates of average spending by transport mode may or may not be true given the small sample sizes involved for cyclists, it is a huge and unjustifiable leap to claim that those who switch from public transport to cycling will reduce their spending to the level of the average cyclist.

Moreover, the analysis then makes a third even more egregious assumption that this reduction in retail spending is just lost to the entire economy. Individuals are assumed to effectively burn the money they would have otherwise spent: another unjustifiable assumption that underpins the implausible estimates of economic impacts and job losses.

Any decent economic analysis will attempt to consider both the costs and the benefits of a proposal. This analysis does not even mention the potential economic benefits of reduced congestion, which the Department of Transport estimate will exceed €100 million per year within the canal and €800 million per year across Dublin more generally by 2030.

Economic evaluations have an important role to play in informing public debate around the trade-offs between competing objectives. But such evaluations need to be based on good data and realistic assumptions, appropriately caveated where either is lacking.

If this analysis were submitted as an undergraduate dissertation to the university I teach in, I would fail it. It shouldn’t have any role in informing the debate around the Dublin City Transport Plan.

Barra Roantree is Assistant Professor & Director of the MSc in Economic Policy at the Department of Economics, Trinity College Dublin