Many people incorrectly assume these signs only apply to large trucks. These signs are commonly found in the left lane along 400 series highways containing 3 or more lanes and apply to any vehicle displaying plates with black letters/numbers.
The average full size pick-up truck is around 5.8 meters (19 feet) long. Add any type of trailer and you will be over 6.5 meters (21.3 feet).
Reference: Highway Traffic Act s.186