In jurisdictions where use of headlights is optional when visibility is good, drivers who use headlights at all times are less likely to be involved in a collision than are drivers who use headlights only when visibility is poor. Yet Highway Safety Department records show that making use of headlights mandatory at all times does nothing to reduce the overall number of collisions.
The statement 'Only very careful drivers use headlights when their use is not legally required' most helps to resolve the apparent discrepancy in the information above.