What does a driver need to do before merging onto the highway?

Before merging onto the highway, it is crucial for a driver to increase their speed to match the flow of highway traffic and check mirrors for safe gaps. This action enhances the safety and efficiency of the merging process. By adjusting speed to adequately match that of the vehicles already on the highway, the driver can enter the roadway smoothly without causing disruptions or creating hazardous conditions. Additionally, checking mirrors for safe gaps ensures that the driver can merge safely into a position that does not interfere with the existing traffic.

The other choices do not align with safe merging practices. Yielding to all vehicles may not always be possible or necessary if gaps are available, stopping could cause a dangerous situation, turning on hazard lights is generally not appropriate for merging, and while signaling is important for communication, it is not enough on its own without also ensuring that speed and gaps are addressed. Thus, option A encompasses the key actions necessary for successful merging onto the highway.