In the Highway Safety Manual, what happens to the predicted average frequency of single-vehicle crashes on urban/suburban roads as traffic volumes increase?

• A. Frequency declines as volumes increase.
• B. Frequency increases linearly without bound.
• C. Frequency continues to increase but at a decreasing rate.
• D. Frequency remains constant.

Answer: (C)

The relationship is sublinear: as traffic volume increases, the predicted average frequency of single-vehicle crashes on urban/suburban roads rises, but the rise slows down. In the Highway Safety Manual, Safety Performance Functions describe crash frequencies as a function of volume, and for single-vehicle crashes on these road types the curve is concave. A common way to express this is with a function like E[crashes] = a × volume^b, where the exponent b is between 0 and 1. This means doubling volume leads to more crashes, but not twice as many—the extra crashes gained with each additional unit of volume diminish as volume grows. The intuition is that more vehicles mean more exposure, yet higher volumes bring factors (like slower speeds, driver adaptation, and congestion effects) that reduce the marginal increase in crashes. The other options imply no growth, linear growth without bound, or a decrease, none of which align with how exposure to risk behaves in practice.