A bypass road is a complex archeological and social fact.

Most of them embody the principle that the shortest way between two points, in time, is no longer the shortest distance in space. The reason is simple: the shorter roads are crowded and slow.

Attitudes about bypass roads also vary across time and place. Merchants often object to them, fearing that their greater traffic efficiency will translate into fewer customers stopping by. Residents often applaud them, hoping that noise and pollution will be moved further away. Sometimes these roles reverse over time.

Madrid built its M-10 ring road in the 19th century to route traffic, especially commercial traffic, out of the coveted central residential area. Then in the age of the mass automobile it built the M-20, and in the 1970s the M-30, a 32-kilometer road around the expanded almond-shaped center. One million people live inside the M-30 perimeter today, which means the majority of the city’s residents live outside. There is an M-40, and a major project to turn part of the M-30 into a tunnel and create a 100-hectare park of prime public real estate.

Many cities have similar tales. The bypass or ring road ends up becoming a new demographic baricenter, or, in other words, merchants morph into residents. In some U.S. cities, such as Des Moines and Omaha, the interstate skirt urban core while and the bypass traverses their centers. Beijing has no less than five ring roads today.

History marches on. Marylebone Road in London was built as a bypass between Paddington and Islington in 1756. Today it is part of the British capital’s Inner Ring Road – meaning it demarks the point where congestion charges go into effect – and is home to no fewer than six Tube stops. And it’s often forgotten today, but Italy’s famous A1 highway, the Autostrada del Sole, was initially A1 from Rome to Milan and A2 from Rome to Naples. Unification of this iconic and axial highway’s name came when a bypass road around the Grande Racordo Anulare (GRA), itself a ring road, was built.

Bypass roads can also end up being used in ways quite alien to their original design. The GRA itself provides an example, as it was deployed as a border point in an anti-pollution initiative to delimit access for older vehicles. A chronological rendering of Rome’s postwar urban development would show that the GRA, built to enhance the flow of traffic, especially for longer-haul driving, served in fact as a guide to where to build. Today, 26 percent of municipal residents live outside the GRA, up from only 18 percent in 1999. Daily vehicle traffic has risen by 50 percent, causing the loss of 135 million person-hours which alone amount to 1.5 billion euros in economic damage every year – a hefty sum that would warrant investment in a new bypass road.

And yet it’s worth noting that this is not all merely the inevitable corollary of rapid population growth. Research led by Armando Montanari of La Sapienza University calculate that while Rome’s population increased 12-fold from 1871 to 2001, the urban space its people occupy has increased by a factor of 52, more than four times the growth rate.

It’s no surprise to learn that Rome has far more cars per capita than Paris or London, or indeed compared to the rest of Italy. Nor is it a surprise to learn – from Rome’s new General Plan for Urban Traffic – that people are relying less on public transport over time, as the network struggles to adequately serve such dispersion.

While the case for more bypass roads in Rome appears strong, it’s also evident that the city is not suffering from excess density. Indeed, traffic engineers everywhere know the concept of “induced demand” first articulated in 1969 by J.J. Leeming. The theorem is simple: adding more road capacity simply encourages more traffic and so increases the amount of congestion.

The ratio is a neat one to one over the long haul, according to a detailed analysis of multiple U.S. cities between 1980 and 2000 done by Gilles Duranton of the University of Pennsylvania. There’s even evidence – from Paris, San Francisco and quite famously from Seoul - that the rule works in reverse. Eliminating roads does not create the traffic apocalypse many fear.

Research, along with local experiments, offer a wealth of promising insights and sweep away some groundless presumptions. Introducing higher peak-time parking charges in San Francisco, turned outto benefit retailers as more customers were sure they’d be able to park. And residents’ overall spending on parking decreased thanks to lower off-time charges.

Congested roads may also reflect factors that have nothing to do with engineering. Belgium has some of the worst traffic bottlenecks in Europe and that is partly due, according to the Organization for Economic Cooperation and Development, to very high value-added taxes on real estate, which impede people’s ability to move when their job locations change. House prices are also a factor behind younger Roman households moving further from the center, according to Montanari.

Given the conundrum of induced demand and the host of exogenous factors that influence the fate of a bypass road, assessing whether one is needed probably depends on what is identified as the goal.

Ever since Emperor Trajan ordered the first bypass road to solve a legendary bottleneck along the Appian Way, the idea has been to make business more efficient. U.S. bypass routes on the interstate system were originally called truck roads.

Yet ironically it is mixed-use urban landscape – districts that host more than one function and draw in different people with different purposes at different times - that enjoy the cachet of being vibrant places. The efficiency goal of a bypass road must be weighed against other and ultimately paramount values.

That’s true also because the future has always been full of surprises. Today there is great enthusiasm about self-driving cars. And it is true that perhaps technology will allow major revolutions such as reducing the space between vehicles moving at speed, offering major congestion relief with no extra physical roads. Yet one unexpected result of a “smart” motorway reform in England – stripping out hard shoulders to increase capacity and cut congestion – has led to a whopping 37 percent of drivers categorically avoiding the new lane for fear they’ll be blocked by a broken-down vehicle.

Authorities now plan to build more frequent lay-bys, a costly intervention if executed.

So some caution is in order. After all, the advent of the railway was so domineering in Britain that authorities let turnpikes fall into disrepair. It took decades before cars were taken seriously.

Given the powerful network effects that factors such as house prices, climate change, air pollution and e-commerce – all of which are the subjects of serious international scrutiny today – have on the destiny of any transport solution, including roads, major decisions have to be taken in an inclusive and integrated way.

Traffic around Mestre, near Venice, was infamous for decades due to bottlenecks caused by capacity running at three times the road’s designed capacity. The new Passante bypass, opened in 2009 as a surface road rather than the costly tunnel desired by local towns, is obviously a welcome and long-overdue solution.

The capacity squeezes elsewhere are not always so extreme. Often innovative alternatives might be more adept at achieving identified transport goals.

That said, some proposals may be too clever by half. For example, several city bike-sharing programmes in China have led to abuse, including random disposal of the bikes which paradoxically was enabled by the use of “smart” apps and locks that made docking stations unnecessary. One company is now paying people who find abandoned bikes and pedal them back into circulation.

In general, transportation infrastructure decisions require sifting through a host of concerns and arriving at a viable compromise. The price signal is probably best placed to achieve such an outcome. Not only can prices work to negotiate tradeoffs, but they have a special role in the world of public network goods. As Brian Taylor, a professor of urban planning at the University of California, Los Angeles, notes, the evidence suggests that road-and-transport pricing schemes can off the Lemming prophecy at the roots.

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