It is not possible to name one structure as the longest highway in the world because there are many types. Instead, the longest for each variant will be listed here.
The longest would be the Pan-American Highway. As of 2005, it is almost 25,000 kilometers long. It links numerous countries in the Americas. However, it is not continuous. Due to heavy rainfall and rugged terrains, there is a gap at the southeast of Panama.
The longest point to point would be the Trans-Canada Highway at 7,821 km (4,857 mi) long. It runs from east-west along southern Canada up to the country’s populated sectors. It hooks up several urban centers. It also crosses practically all the provinces and capital cities.
When it comes to circuits, the longest highway in the world would be Australia’s Highway 1. It is more than 20,000 km (12,427 mi) long. The expressway is so long it goes through almost all of Australia’s coastline. All of the country’s capital cities are linked except for Canberra, which is inland.
The largest highway system would be the US, which has 6,430,366 kilometers (3,995,644 mi) of roads. The busiest is Highway 401 at Ontario, Canada. The average volume is over 500,000 vehicles daily.
The widest is Katy Freeway at Houston, Texas. It has 26 lanes. But these lanes are separated into HOV lanes/frontage roads and general use. This limits the traffic flow.
If the maximum number of through lanes is considered, the widest would be Interstate 5 by the two-mile by California State Route 56 and Interstate 805. It is 22 lanes wide.
It is the Karakoram Highway along China and Pakistan. It is 4,693 m/15,397 ft high.
Expressways became necessary as automobiles became popular. Expressways would become prevalent around the early 20th century. The first limited access road was developed in 1922 in Italy. Building of the Bonn-Cologne began in 1929.
The first UK motorway was inaugurated in 1958. In 1056, the Federal Aid Highway Act allowed for the building of 66,000 km (41,000 miles) of Interstate Highways. These would be completed over a 20 year span. Other countries have adopted similar policies.
It should be pointed out that most of these roads are under constant development. Thus, the length of the longest highway in the world can change anytime. When they are being repaired, some parts may also not be accessible or be shut down.