Highway

A highway is a fundamental component of land transportation. In its broadest sense, a highway is any public or private road or public way on land. This can include major roads or even smaller rights of way.

In the United States, the term specifically refers to major roads designed for motor traffic at considerable speeds. These are often called controlled-access highways. This term describes roads that limit where drivers can enter or exit to keep traffic flowing. Different countries use different names for these high-speed routes, such as motorways in the UK or Autobahns in Germany.

Modern highways are engineered with specific features to increase capacity, efficiency, and safety. One common feature is the dual carriageway, which provides two or more lanes on each side of the road.

Another important design is the grade-separated junction. This allows different roads or modes of transport to cross each other at different heights, preventing direct collisions. Many highways also reduce the number of locations where users can access the road. These design choices are most common on motorways, which are high-speed, limited-access roads.

Engineers use several metrics to measure how well a highway performs. One measure is free-flow speed, which is the speed most drivers choose when traveling alone without many restrictions. This speed depends on the driving environment and can change depending on the time of day. Another metric is running speed, which is the average speed of a vehicle while it is in motion.

Finally, engineers look at average travel speed to determine a highway's level of service. They also measure control delays, which are pauses caused by traffic control devices like signals at intersections.

Highways have changed significantly throughout human history. Originally, highways were paths used by people on foot or on horses. As technology progressed, these routes accommodated carriages and bicycles. Eventually, advancements in road construction allowed for the use of motor cars.

In the 1920s and 1930s, many nations began investing heavily in highway systems. They did this to encourage commerce and to strengthen national defense. This era marked the beginning of the massive, interconnected networks we use today.

Significant milestones in highway history include the development of the first controlled-access roads. In 1911, the Long Island Motor Parkway was completed in New York. While it featured modern elements like banked turns and guard rails, it was not a true controlled-access highway because traffic could still turn left across oncoming lanes. Italy became the first country to build controlled-access highways reserved only for fast motor vehicles. The Autostrada dei Laghi, which connected Milan to Lake Como and Lake Maggiore, was inaugurated in 1924.

Later, the German Autobahn became the first high-speed, limited-access network in the world. In the United States, the Federal Aid Highway Act of 1956 allocated $25 billion to build the Interstate Highway System over 20 years.

Highways are classified in different ways depending on the region. In the United States, the Federal Highway Administration categorizes them by accessibility and mobility. Principal arterials, such as freeways, have high mobility. Collectors balance mobility and accessibility, while local roads provide high accessibility to specific areas.

In Europe, the European Commission uses similar categories. They define arterial routes for through traffic, distributor roads for moving between areas, and access roads for reaching specific properties. These classifications help planners understand how a road functions within a larger network.

Global highway networks vary greatly in size and scope. Australia is home to Highway 1, which is the longest national highway in the world, traveling almost entirely around the continent.

China possesses the largest highway network on Earth, followed closely by the United States. Some highways are so large they span multiple countries, such as the Pan-American Highway or various European routes. Some major routes even include ferry services to cross large bodies of water, such as US Route 10, which crosses Lake Michigan.