Would the Welsh economy be weaker without an M4 Relief Road?
So would the Welsh economy be weaker without the M4 Black Route - or for that matter without any M4 upgrade?
That question is even more pertinent as Labour and Plaid Cymru talk about what they can and cannot agree on.
Much of the so called "economic" debate has been about its cost - whether we can afford £1bn, and whether the cheaper Blue Route would be a better option .
That is a very different question from whether the Welsh economy would be stronger with it, and weaker without.
There is little disagreement that the M4 between Cardiff West services and Magor, east of Newport, is regularly congested.
And that slows down the A48M and the A470.
There's also little disagreement that the M4 is busiest around Newport, partly because of the unusually large number of junctions which encourages local people use the motorway as a route between the various suburbs of the city.
To really understand the problems we need to look at where employment in south east Wales has grown up over the last twenty years.
Much of it has been developed on new sites along the motorway.
It was part of the old Welsh Development Agency's policy in the late 1980s and early 90s to build big business parks, easily accessible by car.
Imperial Park, Celtic Springs, Cleppa Park and Cardiff Gate are all examples of that.
Many thousands of commuters travel between the Severn Bridge and at least as far west as Swansea to work for companies on those parks.
These developments, and the work they offer, have also been a magnet to people living in the Gwent and Rhymney valleys who may have found it a challenge getting work closer to home.
Significantly, the Welsh Government's document arguing for the Black Route clearly shows that the highest concentration of traffic is not at Brynglas but at Tredegar Park, where the main road from the Gwent valleys, the M4, the A48 and a host of industrial parks all meet.
The cheaper Blue Route would take westbound traffic off the M4 east of Newport but it is hard to see how it would affect the many thousands who travel south towards the motorway.
The big question in terms of easing congestion is how much the Metro integrated road-and-rail transport network will change commuters' patterns.
It partly depends on whether the full Metro plans are implemented, how well and how quickly.
Importantly, the proposed network does include a station at Cleppa Park - the centre of the family of business parks to the west of Newport - and other stops that may tempt people out of their cars.
There is also a station planned for the Royal Gwent Hospital.
If we had a public transport system that carried people from where they tend to live to where the work is - and where they want to go for shopping and hospital visits - then would we need a whole new motorway?
The economic argument for the M4 Black Route centres on time and money lost due to congestion, the poor image that gives of Wales as a modern place to do businesses, and the theory that concentrations of economic activity tend to multiply more quickly than if businesses are more isolated.
Wales' economic weakness centres around having too high a proportion of low-skilled low-value jobs, not enough headquarters, and not enough innovation .
Perhaps the biggest question is not about the price of an upgrade of the M4, but what Wales will get from it.
It's not really about £1bn or £380m - it's about what we get from that.
How much would either route answer the really big challenges of the Welsh economy?
Would Wales become home to more research and development, for instance, with speedier travel along the M4? And would that be the magic factor for encouraging new or growing tech firms to be headquartered here?
That's what Wales really needs.