Two short lengths of road are about to open at a combined cost of £1bn. They represent a last hurrah for expensive road projects in an era of cost-cutting.

At a time of austerity, the idea of spending more than half a billion pounds on a five-mile stretch of road might seem strange to some. But the M74 extension is about to open in Glasgow at a cost of £692m, which works out at £138.4m per mile or £78,000 a yard (£86,500 a metre).

The elevated six-lane highway pushes through the heart of Glasgow like some Corbusian vision of the machine city. For a few weeks it will be the second most expensive road per mile in the UK - after Limehouse Link, in London's Docklands.

But in July its silver-medal status will be trumped by the even more costly A3 tunnel at Hindhead in Surrey. A four-mile section of dual carriageway is being added, but the vast bulk of the £371m cost is concentrated on the 1.2 mile (1.9km) tunnel. It is the longest road, under land rather than a river, in the UK, costing around £300m, equivalent to £142,000 per yard (£155,000 per metre).

It's no surprise these projects are expensive, says Geoff French, vice president of the Institution of Civil Engineers.

"There's a huge cost penalty when you put a road up in the sky or down in the ground," he says.

A mile of new motorway costs on average £30m, according to the Highways Agency. As a rule of thumb, an elevated road costs 10 times more than one on the flat, says French.

The M74 passes over roads, railways and the River Clyde and its longest bridge, the 2,500ft (750m) Port Eglinton Viaduct, spans the West Coast Mainline railway.

"The incredible thing is how close to the centre of Glasgow it is," says Sir Peter Hall, Bartlett professor of planning at University College London. "Some of the exits will be five minutes to the city centre."

Why our ancestors loved road-building Jonathan Glancey, architecture correspondent, the Guardian The Romans saw empire as a physical construct and spent three-quarters of their GDP on infrastructure. Wherever they set up a colony they'd build a city and connect it to the next one. This was 2,000 years ago but they thought nothing of building 50-mile aqueducts. In the 19th Century the Victorians built colossal great sewers and truly beautiful dams and aqueducts. The cost must have been unbelievable as some of the water infrastructure was done in granite. Today everything's changed. Unlike Germany, Britain has decided to run its economy on shopping rather than building things. People are blase - more excited about the latest app than some breathtaking infrastructure project. Today it's a "so what"?

That affects not just the engineering but buying up land. The compulsory purchasing of property in the M74's path cost £200m, according to Transport Scotland. That is on top of the expense of running planning consultation and public enquiries.

An underground road costs even more - roughly twice that of an elevated one, French says. Such arithmetic saw Boston spend around $20bn (£12.5bn) on its Big Dig that rerouted the city's main highway into a 3.5 mile (5.6km) tunnel.

Completed in 2007, it was controversial for being the most expensive highway in US history. Perhaps with that in mind, New Jersey Governor Chris Christie last year cancelled an $8.7bn (£5.5bn) rail tunnel under the Hudson river at Newark.

But sometimes there is no alternative to going underground. Currently, the A3 dual carriageway from London to Portsmouth is forced to narrow to a single lane in each direction at the Devil's Punch Bowl beauty spot, creating a notorious traffic blackspot.

The tunnel allows the road to expand to four lanes by digging up to 195ft (60m) under Hindhead Commons. "Building a dual carriageway on the surface would have caused great damage," says Rob Fairbanks, director of the Area of Outstanding Natural Beauty, which contains the Devil's Punch Bowl.

The Hindhead scheme is made up of two separate tunnels, one for each carriageway, which are linked every 330 ft (100m). More than 4,000 people worked on the project and 26 million cubic feet (737,000 cubic metres) of earth have had to be excavated during the tunnel's construction. It is equipped with lights twice as bright as the floodlights at Old Trafford.

Image caption Boston's 3.5 mile "big dig" proved hugely controversial

The most expensive road per mile is the Limehouse Link. The 1.1 mile (1.8 km) tunnel in London's Docklands opened in 1993 at a cost £293m. Adjusted for one measure of inflation that would be £445m or £230,000 per yard (£250,000 per metre). It was designed and built in seven years and at the time was the second biggest engineering project in Europe after the Channel Tunnel.

"It was almost insane," says Sir Peter Hall, Bartlett professor of planning at University College London. "But Margaret Thatcher would stop at nothing to get the Isle of Dogs developed." The price tag can be explained by the fact it had to avoid other tunnels and a river basin and incorporate a junction within the tunnel.

The small but expensive projects demonstrate the way priorities in planning and transport have changed over the years. They are local technical fixes rather than the national networks of earlier decades.

The heyday of British motorways was the late 50s to the 1980s when new roads summoned up the idealistic notions of speed and social mobility.

The art of creating a road Joe Moran, author of On Roads: A hidden history Building a road is a lot more complicated than you think. People assume it's just laying a piece of tarmac. But it's a complex piece of architecture that's always open to the elements. It has to pay attention to geology and nowadays to ecology. But when the M1 was built in the late 1950s it went straight through the countryside like an American freeway. It was constructed at an average speed of a mile every eight days and a bridge every three days. But there were no real objections. In those days there was a completely different attitude. In 1966 Transport Minister Barbara Castle described motorway interchanges as "the cathedrals of the modern world". And in 1972 - the year that Spaghetti Junction opened - Britain clocked up 1,000 miles of motorway. But the atmosphere soon changed and rather than engineering excellence Spaghetti Junction became a symbol of getting lost. Later on came John Major's much-mocked cones hotline, summing up the annoyance and frustration that motorways cause.

The advent of the road protest movement - personified by Swampy at the Newbury bypass protests of the 1990s - showed that attitudes had changed. Governments saw roads as potentially controversial and took seriously the arguments against.

Nowadays the motorway is more likely to be associated with long tailbacks than speed, as summed up by Chris Rea's song about the M25: "This ain't no upwardly mobile freeway, oh no, this is the road to Hell."

Indeed, road projects like the M74 extension and A3 tunnel may never be repeated. The M74 is a "relic" to a bygone age, says Prof Hall. Nowadays, most city councils subscribe to the view that urban motorways fracture communities rather than aid economic development.

Expensive tunnels are also out of favour. A £540m tunnel putting the A303 at Stonehenge underground was scrapped in 2007 by the Department of Transport.

Today the Highways Agency says it has no more major motorway or tunnel projects in the pipeline.

The future is low cost tweaking known as "motorway management" - using electronic speed limit displays and the hard shoulder to vary traffic flow.

Jimmy Savile may have been ahead of his time in the 1980s when he claimed: "This is the age of the train." For the major infrastructure projects of the coming decade look to be rail.

The government is currently consulting on the first section of High Speed 2, a £17bn railway from London to Birmingham.

Some will be harking back to the idealism of the motorway age, but the protest movement is already gathering pace.