What is the main characteristic of a spiral curve in road design?

The main characteristic of a spiral curve in road design is that it features a uniformly changing radius. This design allows for a gradual transition between a straight section of roadway and a circular curve. The gradual change in radius helps vehicles to adjust their steering and maintain control as they move from a straight path into a curve, thereby enhancing safety and comfort for drivers.

Unlike circular curves, which have a fixed radius throughout, spiral curves begin with a larger radius and progressively tighten. This feature is particularly beneficial in minimizing lateral acceleration experienced by vehicles, thus contributing to a smoother driving experience compared to abrupt transitions found with simple circular curves. Additionally, the uniform change in radius supports better sight distances and improved visibility for drivers compared to fixed-radius curves.