What is a key characteristic of active railroad crossings?

Active railroad crossings are specifically designed to alert drivers to the presence of approaching trains and to enhance safety at these intersections. The key feature of these crossings is that they are equipped with warning signals, such as flashing lights and bells, as well as physical barriers like gates that lower to block the road when a train is approaching. This combination of auditory and visual alerts, along with the gates, provides multiple layers of safety for motorists and pedestrians by clearly indicating when it is not safe to cross.

In contrast, railroad crossings that are marked by basic signs only are typically considered passive crossings, where there are no automatic signals or barriers to warn drivers of oncoming trains. It's also important to note that active railroad crossings are not limited to any specific geographic areas; they can be found in both urban and rural settings. While some crossings might not require vehicles to stop, particularly if no train is present or if the crossing has special signals, these do not characterize active crossings. Overall, the presence of warning signals and physical barriers is what distinctly identifies an active railroad crossing, making it the correct answer.