Final answer:
Best practice when responding to a highway incident is to park immediately behind the first unit on the scene when you are a secondary unit and use cones or flares to warn traffic.
Step-by-step explanation:
The best practice when responding to a highway incident is to park immediately behind the first unit on the scene when you are a secondary unit. This allows for a coordinated response and ensures that the secondary unit does not block additional traffic or impede the progress of the first unit. Parking on the same side of the road as the incident may cause congestion and hinder the movement of emergency vehicles. Using cones or flares to warn traffic of the incident is also recommended, as it provides a visual signal to drivers and helps create a safe perimeter around the incident.