Final answer:
The risk of hydroplaning increases at speeds above 35 mph. Air drag significantly impacts vehicle performance at highway speeds, and the most fuel-efficient cruising speed ranges between 45-50 mph. The 1970s oil crisis led to speed limit reductions for increased safety and fuel efficiency.
Step-by-step explanation:
The chances of hydroplaning generally increase as the speed of the vehicle increases. While there is no single speed at which hydroplaning is guaranteed to occur, because it depends on various factors such as tire tread depth and water depth, it starts to become a greater risk at speeds above 35 mph (56 km/h). As speed increases, the tires may be unable to disperse water adequately, which can lead to a layer of water building up between the tires and the road surface. This can result in a loss of traction and potential loss of vehicle control.
According to studies by NASA and other organizations, air drag significantly affects vehicle performance at highway speeds. Over half of the power of a car is used to overcome air drag at these speeds. The most fuel-efficient cruising speed is between 70-80 km/h (about 45-50 mi/h), and during the 1970s oil crisis, speed limits were reduced to about 90 km/h (55 mi/h) to improve fuel efficiency and safety.