In this day and age, mine and construction site require all vehicles to have an identification number that is clearly visible. Some sites insist that the vehicle ID sticker is also reflective.
These fleet numbers can even assist your business in managing a vehicle for maintenance requirements or identifying a vehicle reported as stolen or engaging in unsafe practices.
Generally, the lettering needs to be 150mm high and may be on a reflective or non-reflective backing. Other sites also require the vehicle to have reflective striping along the sides and rear of the vehicle. Generally these are White or Fluorescent.