What Is a Forward Curve?

Identical curve refers to two adjacent curves that turn to the same direction. In the road design, when a short straight line is connected between the curves, the length of the short straight line should not be less than 6 times the length (m) of the calculated speed (km / h), so that the car can operate and maintain the linear continuity. In particularly difficult areas, the calculation of highways with a speed of less than or equal to 40 km / h, the length of short straight lines should also be calculated not less than 2.5 times. When circular curves are directly connected, they are called complex curves in the same direction; when there are spiral curves connected between the curves, they are called oval curves or C-shaped curves.

Identical curve refers to two adjacent curves that turn to the same direction. In road design,

In the railway design, in order to ensure the smooth running of the train and the convenient maintenance, the two curves should not be directly connected, and a straight line must be inserted ^[1]

In the alignment design of the route plane, sometimes two continuous same-direction curves need to be set due to various objective conditions. The common practice is to connect the two curves at a relaxation curve rate of 0. For the C curve, when the distance between the two intersection points is too short to set a longer relaxation curve, you need to connect a relaxation curve between the circular curves of the two isotropic curves, that is, the oval pupil line.

Regarding the concept of the "oval" curve, although it has been explained in the specification, the specific measurement method has not been described in detail. Because its calculation is more complicated, it is rarely used by designers ^[2] .