A decrease in speed causes an increase in which of the following?

When there is a decrease in speed, the effect on traction can often be positive, making it the correct choice in this context. Traction refers to the grip that a vehicle's tires have on the road surface. As speed decreases, the force exerted by the tires on the road can increase, which enhances the vehicle's ability to maintain contact with the road. This improved contact can be crucial during maneuvers like turning or stopping, as it reduces the likelihood of skidding or losing control.

In contrast, acceleration is related to the rate of change of speed, and a decrease in speed does not inherently lead to an increase in acceleration; in fact, it can lead to the opposite effect. Fuel efficiency can also be influenced by speed, but it's typically optimized within a certain speed range rather than simply improving with a decrease in speed. Road safety can be affected positively by a lower speed in some circumstances, but it is a broader concept that encompasses various factors beyond just speed alone. Therefore, the most direct and relevant impact of decreased speed is its positive effect on traction.