[karaterobot]

Depending on where you are in the U.S., a "cross only at crosswalks" sign is necessary because all intersections are considered to act as crosswalks even when there are no solid white lines. Pedestrians can still cross, and vehicles should yield the right of way. That sign could indicate an exception to the rule, due to some dangerous, non-obvious condition that makes it unsafe for pedestrians to cross the road.