Controlled-access highway

• 1. Introduction

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  In 1924, Italy opened the Autostrada dei Laghi, a road linking Milan to Lake Como and Lake Maggiore, and changed the way the world thought about moving from one place to another. Devised by a man named Piero Puricelli, this single-lane-per-direction road was the first in the world reserved exclusively for fast motor traffic. No pedestrians. No horse-drawn carts. No intersections. Just speed. The controlled-access highway had arrived. What drives the engineering logic behind these roads? How did a modest Italian motorway spark a global transformation in how nations are stitched together? And what happens when a road designed for efficiency becomes a force that divides the very communities it was meant to serve?

• 2. The Logic of the Freeway

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  Piero Puricelli's original autostrada contained only one lane in each direction and no interchanges at all. That seems modest by today's standards, but the principle it embodied was radical: separate fast traffic from everything else, eliminate the points of conflict, and the road almost governs itself. A fully controlled-access highway provides unhindered traffic flow by removing signals, intersections, and direct property access. Opposing lanes of traffic are divided by a median strip or crash barrier, which eliminates head-on collisions at a stroke. Where other roads cross, underpasses and overpasses take the place of intersections. Drivers join and leave via slip roads, or ramps, which allow them to gradually match speed with the main carriageway. The result is a road where traffic does not need to stop. This is not just a matter of convenience. Motorways, accounting for more than a quarter of all kilometres driven in Europe, contributed only 8% of total road deaths across the continent in 2006. The injury crash rate on Germany's autobahn network is very low, even though some sections carry no speed limit at all. The trade-off is real: autobahns with no speed limit but a recommended maximum of 130 km/h are 25% more deadly than those with an enforced limit, according to the European Transport Safety Council. Germany later introduced 130 km/h limits on several previously unrestricted sections, and those sections saw deaths fall by between 20% and 50%.

• 3. Origins and First Movers

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  Germany had its own early attempt at a high-speed road. In Berlin, the AVUS, planned from 1907 and begun in 1913, was a closed circuit with two long straights where vehicles could reach top speed before looping back. It opened in 1921 and spent most of the next two decades as a test track and racing venue before being connected in 1940 to the Reichsautobahn national network. That national network had itself been proposed in the 1920s, stalled by the Great Depression, and then built using tax money in the 1930s. The first controlled-access autobahn without speed limits, running 30 km between Cologne and Bonn, was completed in 1932. Konrad Adenauer, then mayor of Cologne, presided over its opening. On the other side of the Atlantic, New York led North America. The Long Island Motor Parkway opened in 1908 as a private venture, making it the world's first limited-access roadway. It had banked turns, guard rails, and reinforced concrete tarmac. Drivers could still turn left across oncoming traffic, however, which meant it fell short of the full controlled-access standard later codified by federal guidelines. The Bronx River Parkway went further: it was the first North American road to use a median strip to separate opposing lanes, and it was built through a park with intersecting streets carried over bridges. Britain, heavily shaped by its railway tradition, did not build its first motorway until 1958, when the Preston Bypass opened as part of what is now the M6.

• 4. The Word Itself

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  The term freeway has a specific origin. In February 1930, a lawyer and urban planner named Edward M. Bassett first used the word in print. Bassett proposed that roads be sorted into three types: highways, parkways, and freeways. Under his property-law framework, owners of land bordering a highway had rights of light, air, and access. Parkway and freeway abutters did not. The distinction between the two was purpose: a parkway exists for recreation; a freeway exists for movement. The United States government formalized this in the Manual on Uniform Traffic Control Devices, which defined a freeway as a divided highway with full control of access. An expressway, by contrast, is divided but only partially controlled, and may still have driveways and at-grade intersections. That distinction was first drawn by the Special Committee on Nomenclature in 1949, and was later incorporated into federal law under a 1966 statute. Eight states went further and codified the definitions in their own statutes: California, Minnesota, Mississippi, Missouri, Nebraska, North Dakota, Ohio, and Wisconsin. Each state translated the federal language slightly differently. Wisconsin requires a divided highway to be at least four lanes wide before either term applies. Missouri limits both terms to divided highways at least 10 miles long that sit outside the Interstate Highway System. Only Minnesota enacted the exact federal definitions, doing so in May 2008.

• 5. Engineering at Scale

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  Interstate 5 near San Diego carries traffic across a 2-mile segment between Interstate 805 and California State Route 56 on a roadway with a maximum width of 21 lanes, using separate express and local lane systems. Highway 401 in Mississauga, Ontario, runs 18 lanes wide through its intersection with Highways 403, 410, and 427. These are extreme cases, but they illustrate how far the original one-lane-per-direction autostrada has been stretched. European motorways are typically designed to a speed of 100-130 km/h, with horizontal curve radii of at least 750 to 900 metres and maximum gradients of 4% to 5%. Cross sections require at least two through lanes in each direction, each 3.50 to 3.75 metres wide, with a central median throughout. Interchanges the point where two controlled-access roads meet are classified differently in different countries. Germany uses Kreuz (cross) or Dreieck (triangle) for a junction between two motorways, and Ausfahrt (exit) for a point where a driver leaves the motorway system. In the United Kingdom, both are called junctions. One notorious example of a system where the two must briefly abandon each other is the connection between Interstate 70 and the Pennsylvania Turnpike near Breezewood, Pennsylvania, where drivers must exit onto a surface road to transfer between the two interstates. At the other end of the scale, China's expressway network reached 149,600 km by the end of 2019, surpassing the total length of the American Interstate Highway System in 2011.

• 6. When Freeways Cut Cities

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  Construction of urban freeways for the US Interstate Highway System, which began in the late 1950s, led to the demolition of thousands of city blocks and the displacement of many more thousands of people. The United States Congress identified freeways and other urban highways as responsible for most of the noise exposure of the entire US population, beginning in the early 1970s. Urban planning scholars including Drusilla Van Hengel, Joseph DiMento, and Sherry Ryan have argued that properly designed freeways may still fail to expand recreation, employment, and education opportunities equally across ethnic groups, or for residents of particular neighbourhoods. Impoverished communities faced a particular disadvantage: they were less likely to own cars, and less likely to have the political or economic weight to resist construction. Some cities eventually reversed course. Seoul removed the Cheonggyecheon elevated freeway and reclaimed it as a stream and park. Portland closed Harbor Drive. Boston buried the Central Artery. San Francisco demolished the Embarcadero Freeway and Seattle removed the Alaskan Way Viaduct. A different solution has gained ground in some cities: underground freeways. Sydney has five underground urban freeways, Brisbane has three, and Melbourne has two. Melbourne's EastLink freeway was routed underground specifically to avoid destroying an ecologically sensitive area. Brisbane faces physical constraints from the Brisbane River and rapid population growth, and has embraced the underground approach with the Clem Jones Tunnel, Airport Link, and Legacy Way already open, and the East-West Link in planning.

• 7. Safety Numbers and Speed

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  Fatigue is treated as a risk factor specific to motorways, given their monotonous character, though many countries do not formally record it. According to Vinci Autoroutes, one third of accidents on French motorways involve sleepy driving. Seat belt compliance on French motorways stood at 98% for front-seat passengers and 87% for rear-seat passengers, but 23% of those killed were not wearing them. Across Europe, motorway fatalities fell by 41% in the decade from 2006 to 2015, although they rose by 10% between 2014 and 2015 alone. Adjusting for the growth of the motorway network over that same decade, fatalities per thousand kilometres of motorway halved. A University of Barcelona study found that when tolls are applied to a controlled-access highway, some drivers divert to alternative routes, reducing safety on roads not designed for heavy traffic. Two-lane freeways, sometimes called Super two roads, carry a particular risk: their sight distances were designed assuming no oncoming traffic, which makes them genuinely dangerous when used as two-way roads. The European route E4 between Gavle and Axmartavlan in Sweden became known informally as a Highway to Hell. Lethal crash rates there did not fall until a median crash barrier was installed, which converted fatal crashes into non-fatal ones.