The first thing to know about the £15bn promise for road “investment” made by the government on Monday is that it is worth half a billion less than when it was first announced last year.

This is not really an announcement, more a handy pre-election reminder. But with more detail comes more suspicion that there might not be £15bn actual investment at all, and if there is it is still a long way short of what used to be spent before the coalition slashed road budgets.

In June 2013 the coalition published a policy paper, Investing in Britain’s Future, which promised £15.1bn improvements by 2021 to England’s strategic road network run by the Highways Agency. A month later it busily put out a command paper, Action for Roads, which was supposed to detail how that would be spent, promising 52 new schemes, including 221 miles of new motorway lane.

Monday’s re-announcement has been politely described as a “fleshing out” of that original promise, although last year’s version was already 80 pages.

The new Road Investment Strategy, as it is called, runs to several documents and promises 127 schemes (including 1,300 miles of motorway and A road widening), of which 84 are “brand new”. Oddly if all of last year’s 52 projects were still there the total would be 136, so nine “investments” appear to have gone missing in the last 12 months.

More importantly, it is not clear how the two sums of money compare: last year there was to be £15.1bn by 2021, a period of up to eight years; Monday’s announcement promised £15.2bn from 2015-2021, or six years.

Last year’s £15bn is now worth about half a billion less after inflation, so it is unclear if more money has been added or if the Treasury forgot to recalculate it for the press release.

Then there is the question of whether this is “investment” or not: the £15bn almost includes 20 “feasibility study” schemes, and a large chunk of “capital maintenance” - say resurfacing a large section of road, work that is technically replacing like for like but arguably improves it at the same time (especially if it has been allowed to fall into disrepair).

It also includes a lot of schemes sometimes called “smart management” – using remotely controlled speed and diversion signs, ramp metering, and rapid response traffic management officers on patrol, to keep traffic at an ideal flow rate, open hardshoulder lanes to vehicles if necessary, and clear blockages as quickly as possible. In the Action for Roads paper, half of the 26 schemes expected to start before the end of this parliament were in this category rather than traditional road building. There is good evidence this is a cut-price way to reduce congestion, but it is questionable whether everybody would consider some traffic signs and highways police as “investment”.

Even if this is all worth calling investment, does it justify its billing as what the transport secretary, Patrick McLoughlin, called “the biggest, boldest and most far-reaching roads programme for decades”?

The original £15bn to 2021 worked out as about £2bn a year “investment”, whereas the current one equates to just over £2.5bn a year average – though the Highways Agency latest annual report has a graph showing a less even distribution, with annual capital investment falling lower than it is now in the next two years, before rising to more than £3.5bn by 2020-21. This compares to capital spending (including capital maintenance) around the last election of approximately £1.8bn 2009-10 and £2.25bn in 2010-11, in today’s prices. But it also follows a steep nearly 30% decline in overall funding to the Highways Agency since then: from £4.1bn in 2009-10 to £2.9bn in 2012-13. Politicians sometimes argue that historic comparisons are not fair because many A roads have been “de-trunked” - that is downgraded from the Highways Agency to local councils. But local road spending has also fallen at the same rate.

Incidentally, the unheralded news from Monday’s announcement might be that spending on maintenance is going to be less than planned for: Action for Roads talks of another £12bn for maintenance; Monday’s Road Investment Strategy speaks of “more than £10bn” additional money for maintenance of national and local roads. This could be down to the change in time span, or it could be a cut in spending plans.

As so often with these strategies, intended to make the government’s spending plans clearer, the road ahead seems more elusive than ever.