What feature distinguishes active railroad crossings?

Answer: C — They have a traffic control device present

Active railroad crossings are characterized by the presence of traffic control devices such as flashing lights, bells, or gates. These features are designed to alert drivers to the approach of a train and ensure safety by managing the flow of vehicle traffic at these potentially dangerous intersections. The use of these active signals significantly enhances visibility and awareness, allowing drivers more time to react and stop if necessary. In contrast, options that mention a lack of traffic control devices or reliance on only advance warning signs would describe passive crossings, which do not provide the same level of active warning or control for oncoming traffic. Similarly, exclusive reliance on cross buck signs illustrates a different type of crossing that does not include the added safety measures found at active crossings. Thus, the defining characteristic of an active railroad crossing is indeed the presence of those critical traffic control devices.