What does one long blast on a police whistle signal for?

A long blast on a police whistle is universally recognized as a signal to stop traffic. This signal is crucial in various situations where it is necessary for vehicles to come to a halt, allowing pedestrians to cross safely or enabling emergency vehicles to pass through intersections without delay. The clarity of this sound ensures that drivers notice it, enhancing road safety and compliance.

The other options do not fit the widely accepted meaning of a long whistle blast. For instance, starting traffic generally involves other signals or gestures, while proceeding with caution would typically require a more nuanced indication. Similarly, giving a warning signal can vary based on context but would not be designated specifically by a long blast. Thus, the use of one long blast on a police whistle directly signifies the instruction to stop traffic.