The New Jersey Turnpike is a toll road in New Jersey; it is among the most heavily traveled highways in the United States (according to the IBTTA, the Turnpike is the nation's sixth-busiest toll road). The northern part of the mainline, along with the entirety of both extensions and spurs, is part of the Interstate Highway System. Construction of the Turnpike from conceptualization to opening took 23 months, from 1950 to 1952. The Turnpike has 12-foot (3.7 m)wide lanes, 10-foot (3.0 m)wide shoulders, 13 rest areas named after notable residents of New Jersey, and unusual exit signage that was considered the pinnacle of highway building in the 1950s. The Interstate Highway System took some of its design guidelines by copying the Turnpike's design guidelines.