At what speed is it generally impractical to design a compound curve?

• A. Below 25 mph
• B. Between 30 and 35 mph
• C. Above 45 mph
• D. Above 60 mph

Answer: (C)

Designing a compound curve becomes impractical at higher speeds, particularly above 45 mph, due to several factors related to vehicle dynamics and safety. At these higher speeds, the lateral forces acting on a vehicle as it navigates a curve increase significantly. Ensuring adequate superelevation and side friction becomes critical to prevent skidding or losing control.

In compound curves, where two or more circular curves of different radii are connected, the transition becomes more challenging at elevated speeds. The geometry of the curves may not provide a smooth transition for vehicles, which can impact driver perception and comfort. Drivers may also experience increased difficulty in maintaining control, especially when navigating a series of curves.

While lower speeds may allow for smoother transitions and more manageable design requirements, the complexity and potential hazards associated with compound curves at speeds above 45 mph make that range less desirable for practical design purposes. Therefore, the focus shifts toward simpler or more straightforward curve designs to enhance safety and operational efficiency for higher-speed travel.