I believe I have been discussing both sides of the issue.
But we need to consider both sides of this a bit more carefully than the Welsh minister, who, in his explanatory note on the 48 page impact assessment, wrote in 3 lines that he believed the benefits justify the cost, without saying why.
The comparison of the two sides is - to a large extent - nicely summed up in the question that someone cleverly suggested, what do you prefer - that I drive at 20mph past your house and I drive at 20mph past (all your) houses, or that we all drive at 30mph past each others houses. There's the value from you driving at slower speed past my house, and the cost of me driving at slower speed past (all your) houses. I have not agreed the time savings from slower journeys are trivial, but maybe more complicated than application of single unit cost to a bulk of minutes extended.
People mentioned a lot of omitted benefits. I observed in my first post that there are omitted benefits, and people found some more, very valid benefits. A lot of those omitted benefits will be reflected in an increased house value. So I started talking about house prices, because that seemed a nice way of thinking about the value to residents of an environment with slower traffic, albeit always hard to quantify. But I asked, to motivate an order of magnitude debate, how much increase in house price might be more than the time cost of extended journeys that people experience. (And its 50 seconds per journey, not per day, and people typically make at least 2 journeys per day, if they make more than 0).
I have not acknowledged the value of time is trivial. I have said there are difficulties with multiplying a constant unit value by an aggregate total time, because a lot of those might be small. But many might be substantial. We don't even know if someone did detailed modelling to find that out. But if we could do that better, we cannot say that suddenly the value is approximately nothing, we don't have enough information.
I took that 4.6bn npv because it was a number to hand, because I wanted a capital value to compare with house prices. But that was indeed lazy, indeed wrong, of me, as that is actually a net figure, costs minus benefits. If we look at the RIA, the time cost side (30 year npv) has a 6.4bn central case, with low and high at 2.7bn and 8.8bn. The 4.5bn comes from deducting quantified benefits from the 6.4bn. My £110m per year back of the envelope, after multiplying it up for 30 years npv, was nearer, indeed a little below, their bottom end estimate. I had indeed been rather conservative when I made some Fermi estimates to quantify it quickly without everything to hand.
We have talked that adding up lots of small amounts of time might be a problem. But even if we can resolve that issue, I don't think is going to make a quantities like those disappear to nothing.
So, back to the question. If you have a choice between people driving past your house at 20mph, given you have have to drive past other peoples houses at 20mph, or the same with 30mph, which would people actually prefer?
Fwiw, where I live, in this county in SE England where people care a lot about property values, the county council did eventually decide, rather later than some neighbouring counties, that continuous residential areas should all be 30mph, with some exceptions on suitable major roads. That's been in place for maybe 10-12 years now, and the continuously inhabited part of my road, about a quarter of its length, was reduced from derestricted to 30mph in that section. As I drive along the local 30mph roads, pretty much nearly all of them, I do think of it as a social compact. I drive here at 30mph, you drive past mine at 30mph. But the way many people actually drive past my house, especially in the rush hour, they don't seem to think that a social compact even at 30mph is one they want to adhere to. Perhaps that is what in part makes me question whether they really would sign up to a social compact at 20mph. But if it was really enforced, I wonder what they really would prefer. I suspect, given the great extent of 30mph suburbia in this part of the world, that journey time extensions might be larger.
Btw, the DfT has been commissioning broader 20mph studies on places where they have been implemented - many have been quietly implemented over the years, with relatively little fuss. Here's
one from 2018. There's over 200 pages of it. At a glance, it notes large evidence gaps on the size and value of many of the impacts; no evidence of any casualty reductions in 20mph areas, though needs reassessing after being in place for 5 years; but general broad support for them among residents who live in them. So to the extent there are loud objections, maybe it is just the usual cake-ism, people expect to benefit from 20mph zones where they live while driving everywhere else without being restricted by them, just like people expect to benefit from large public spending while paying low taxes. Perhaps the Welsh could have implemented most of this in quieter ways without attracting so much attention.
But this still leaves interesting questions of how to make safety decisions, which we don't seem to have got much further on.