When a vehicle is about to enter traffic, how do we alert it to our presence if necessary?

Correct answer: By flashing the headlights without dazzling the other driver.

If the driver entering traffic may not have seen you, you may give brief flashes of your headlights without blinding them, to prevent a possible accident. Headlight flashes serve the same purpose as the horn and may replace it, including in built-up areas. You may also use the horn to prevent an accident, so neither type of warning is the only one allowed. In the image, the blue passenger car is entering the roadway from the right side of the street.

Real Decreto 1428/2003, de 21 de noviembre, por el que se aprueba el Reglamento General de Circulación · Real Decreto 1428/2003, de 21 de noviembre, por el que se aprueba el Reglamento General de Circulación