reverse curve
{{Short description|Type of curve on a pathway}}
In civil engineering, a reverse curve (or "S" curve) is a section of the horizontal alignment of a highway or rail route in which a curve to the left or right is followed immediately by a curve in the opposite direction.{{cite book |url=https://books.google.com/books?id=PzA3N6eP4TYC&pg=PA164 |pages=164–179 |title=Railway Track Engineering |author=Mundrey |year=2000 |isbn=9780074637241 |publisher=McGraw-Hill Education}}{{cite book |url=https://books.google.com/books?id=G35kAAAAMAAJ&q=railway+%22reverse+curve%22 |pages=86–88 |title=Route Surveys and Construction |author=Harry Rubey |year=1956 |publisher=Macmillan Publishers}}
On highways in the United States reverse curves are often announced by the posting of a W1-4L sign (left–right reverse curve) or a W1-4R sign (right–left reverse curve), as called for in the Manual on Uniform Traffic Control Devices.{{cite book |url=https://books.google.com/books?id=j98qAQAAMAAJ&pg=PA67 |page=67 |title=Manual on uniform traffic control devices for streets and highways |author=Federal Highway Administration |year=1971 |publisher=U.S. Government Printing Office}}
On rail routes, reverse curves can cause buffer-locking. On the Northeast Corridor in the United States, these also hinder the development of high-speed rail.{{Cite web|url=https://www.trains.com/trn/news-reviews/news-wire/analysis-long-road-ahead-for-improving-northeast-corridor-speeds/|title = Analysis: Long road ahead for improving Northeast Corridor speeds}}