The achievement of such a system in the automobile age required a new form of road. This grew from the parkway, which had many historical precedents but was introduced in its modern form in 1858 with the work of the landscape architects Frederick Law Olmsted and Calvert Vaux for Central Park in New York City. The concept was given further prominence by William Niles White of New York as a part of the Bronx River protection program of New York City and Westchester County. The 15-mile, four-lane single carriageway known as the Bronx River Parkway was built between 1916 and 1925. Protected on both sides by broad bands of parkland that limited access, the highway was located and designed so as to cause minimum disturbance to the landscape. Its use was restricted to passenger cars, and at-grade intersections were avoided. The success of the concept led to the creation of the Westchester County parkway system and the Long Island State Park Commission. More parkways were built in the New York area, including the Merritt Parkway (1934–40), which continued the Westchester Parkway System across Connecticut as a toll road providing divided roadways and limited access.
The success of the parkway system led to the introduction of the freeway, which is a divided highway with no conflicting traffic movements and no access from adjoining properties. In Germany between 1913 and 1921 a group called AVUS had built 10 kilometres (6 miles) of parkway through the Grunewald park in Berlin. Their successful experience led to the world’s first full freeway being built from Cologne to Bonn between 1929 and 1932. In 1933 Adolf Hitler began construction of an integrated freeway network known as the Reichsautobahnen, or “national motor roads,” beginning with the Frankfurt-Darmstadt-Mannheim-Heidelberg Autobahn. One purpose of the program was to alleviate unemployment, but the roads also appealed to German nationalism and had a strong militaristic intent. The entire system included three north-south routes and three east-west routes. The highway provided separate 7.5-metre (25-foot) carriageways divided by a median strip of 5 metres (16 feet). The roads were designed for large traffic volumes and speeds in excess of 150 kilometres (90 miles) per hour, bypassing cities and providing limited access. About 1,000 kilometres (600 miles) were completed by 1936, and 6,500 kilometres (4,000 miles) were in use when construction ceased in 1942.
The viability of the freeway concept in the United States was demonstrated by the Pennsylvania Turnpike. The Pennsylvania Turnpike Commission, established in 1937 to raise funds and build a toll road across the Appalachian Mountains, found an unusually favourable situation in the form of an abandoned railroad right-of-way, with many tunnels and excellent grades over much of the route that allowed the tollway to be completed in 1940 to freeway standards. The turnpike provided two 24-foot carriageways and a 10-foot median with no cross traffic at grade and with complete control of access and egress at 11 traffic interchanges. Its alignment and grades were designed for high volumes of high-speed traffic and its pavement to accommodate the heaviest trucks. The favourable public reaction to this new type of highway provided the impetus for the post-World War II toll-road boom in the United States, advanced the start of a major interstate highway program, and influenced highway developments elsewhere. The Pennsylvania Turnpike, originally running from Harrisburg to Pittsburgh, was later extended 100 miles east to Philadelphia and 67 miles west to the Ohio border, making it 327 miles long. An original feature of the turnpike, later widely copied, was the provision of restaurant and fueling facilities.