The somewhat hidden (non financial) cost seems to have been giving up on some of the existing public transport services - like trams. The Wikipedia article mentions that briefly towards the end.
The trams were already on the way out in the 1950's, they were seen as a thing of the past that were inhibiting the development of the new car-centric urban areas. Several systems closed well before H-day, in the 50's and early 60's. The ones that closed on or around H-day were mainly in Stockholm and in Helsingborg.
In Stockholm, plans had been made as early as the 1940's to gradually replace the tram network with the metro, and that did in fact happen. Today's green and red metro lines do in fact use old tram line alignments to a pretty significant extent (the younger blue line was all new and mostly blasted out of the bedrock deep underground). In 1957 the city council formally decided that the tram networks should be gone by the mid-1970's, at which time most of the infrastructure and rolling stock would have been considered end-of-life. All H-day did was accelerate this by a couple of years. Two suburban lines were kept though and they survive to this day, but at least up until the 1990's their future was kind of uncertain. Things finally changed in the mid-90's, and new light rail alignments started to get built again.
It was not done throughout, but cities with trams with doors on only one side posed a problem. So it is my understanding that when such retrofits would have been to costly, the trams were decommissioned instead.
In Stockholm, plans had been made as early as the 1940's to gradually replace the tram network with the metro, and that did in fact happen. Today's green and red metro lines do in fact use old tram line alignments to a pretty significant extent (the younger blue line was all new and mostly blasted out of the bedrock deep underground). In 1957 the city council formally decided that the tram networks should be gone by the mid-1970's, at which time most of the infrastructure and rolling stock would have been considered end-of-life. All H-day did was accelerate this by a couple of years. Two suburban lines were kept though and they survive to this day, but at least up until the 1990's their future was kind of uncertain. Things finally changed in the mid-90's, and new light rail alignments started to get built again.