- Historic Sites
The Road To The Future
Fifty years ago the builders of the Pennsylvania Turnpike completed America’s first superhighway—and helped determine the shape of travel to come
May/june 1990 | Volume 41, Issue 4
If the time for beginning the project had seemed short, the amount of time allotted to finish it was even more startling: the road had to be substantially complete by May 1, 1940, later amended to June 29 of 1940—a mere twenty months. A job of this magnitude normally would take three or four years to complete. It resulted in 155 contracts’ being awarded to 118 firms, and before long ten thousand employees and $12 million worth of roadbuilding equipment were at work around the clock.
The design and safety standards for the roadway were unprecedented in the United States, although they were similar to those of Adolf Hitler’s hundred-mile-an-hour autobahns, which had been built for military and ideological purposes beginning in 1933. Just ten years before work started on the turnpike, many American highways had been built with a total double-lane width of 16 feet. To keep down the cost of right-of-way acquisition and excavation, the turnpike’s engineers considered using a 20-foot-wide design (that is, two 10-foot-wide lanes in each direction), but because they expected a mixture of heavy truck and auto traffic, they instead agreed on a 24-foot standard. In the tunnels the pavement narrowed to two lanes, each 11.5 feet wide. Engineers also considered, then rejected, another cost-cutting plan to pave two lanes with reinforced concrete and two with asphalt; they used concrete for all four lanes. Concrete has a higher first cost but is cheaper to maintain in the long run. A narrow, 4-foot median strip was scrapped in favor of a 10foot-wide divider.
Of the 160 miles of roadway, 110 miles was on straightaway. Superelevation, or banking, was built into the curves to a very specific end. Many contractors on the job had worked on early paved roads whose curves had been deliberately constructed without banking to discourage higher speeds. This had contributed to skidding and overturning accidents. The superelevation on the turnpike was a carefully chosen compromise: Curves were banked, but not so steeply as to encourage speeds greater than those that would allow a driver to stop within the turnpike’s standard 600-foot minimum rear-end-collision sight distance, which generally assumed a typical top speed of 70 miles an hour.
Another feature related to superelevation was a technique that had been used in railroading for many years: the transition, or spiral, curve. With this alignment design, the highway eased gently from straightaways into curves, so that the sharpest part of any curve was at its midpoint. The practice, reported Roads and Streets in October 1939, “has ... only recently been adopted for state highway work, and has been used by comparatively few states.”
Where curves were required, the sharpness was matched to adjacent highway sections to account for the psychological tendency to maintain a speed once it has been attained. Thus the longer the tangent, or straight stretch, the milder the curve allowed at either end of it. Although straight segments of the autobahns had been built with slight curves designed into the alignment every few miles to stimulate driver awareness, no such rule applied on the turnpike; for most of the distance none was necessary, for the rugged countryside required curves anyway. But some motorists still complain today about the boredom of a nearly 40-mile stretch west of Carlisle that is relieved by just one gentle curve. Later highways have been built to avoid such long straightaways.
Grades were limited to 3 percent to ensure that trucks and, if necessary, military vehicles could use the highway as easily as pleasure cars. The Vanderbilt railroad had been designed for long, heavy freight trains, and its maximum grade was only 2 percent. As a result, the turnpike often diverged from the railroad alignment to strike out on the shorter course permitted by a slightly steeper grade. The transitions from level highway to grades, and over the summits, were designed, like the curves, with minimum daylight and headlight sight-distance standards in mind.
In addition, to give the highway all-weather availability, the route was, where practicable, laid out on the southern exposures of hills to take the fullest advantage of solar heating to clear snow and wet pavement. Some of the excavation required was of heroic proportions. Planners originally envisioned an eighth tunnel near Everett but at the last minute decided to make it an open cut, necessitating the removal of 1.1 million cubic yards of earth and rock. The result was Clear Ridge Cut, which was, at 153 feet deep, the largest highway cut in the nation at that time.
The turnpike’s nine intermediate interchanges, each having its own tollbooths, were built with 1,200-foot-long entrance and exit lanes, always located in the middle of a straight stretch of roadway; at a “sag,” or long, gentle dip; or both. Moreover, the ramps and tollbooths were placed to give motorists leaving the turnpike the advantage of always looking down on the tollbooth - and - interchange layout and getting a bird’seye view of it before entering it, rather than being surprised by coming over the top of a hill and suddenly being confronted with a splitsecond decision. Tollbooths were built across the highway itself at Irwin, the western terminus, and at Carlisle, near the eastern end.