My article on driving lights led to a number of requests to write a follow up article on fog lights. The original question involved vehicles that were driving with four lights on all the time and two of them were not being dimmed for oncoming traffic. Many readers were aware that the extra two lights were fog lights and not driving lights.
What is a Fog Light?
Fog lamps are identified by the SAE F marking on the lens, or a B above the circle with the E in it on European lamps. Lights that meet the latest performance standard will be marked F3 instead of F or B.
Fog Light Installation
B.C.'s Motor Vehicle Act Regulations (MVAR) allows two fog lamps that emit either white or amber (or properly: selective yellow) light. They must be mounted on the front of the vehicle, below the headlamps, but not more than 30 cm below. When you switch them on, the parking lamps, tail lamps, licence plate lamp and, if required, clearance lamps must also illuminate.
However, the Vehicle Inspection Manual used by Designated Inspection Facilities to insure vehicle equipment complies with standards also mentions red rear fog lights. Two are allowed and must be mounted more than 10 cm. away from the brake lights.
The two sets of rules have been out of step for a long time.
When Should You Use Fog Lights?
Despite this, the MVAR says that front fog lamps may be used in place of headlamps if atmospheric conditions make the use of headlamps disadvantageous.
Beyond that, front fog lamps may be used at any time of the day or night and in fact are used as the daytime running lamps on some vehicles.
The MVAR doesn't say anything about the use of rear fog lights. Dan suggests that a good metric to use in deciding when to switch your rear fog lights on or off is "Do I wish the guy in front of me was displaying a rear fog so I could see him better / Do I wish the jerk in front of me would turn off that damn bright red light?".
How to Aim Front Fog Lights
As with driving lamps, fog lamp aim is measured at a distance of 7.62 m from the lamp.
If the beam is symmetric, aim is measured at the center of the top edge of the high intensity area. They must be 100 mm below horizontal with an upper error of no more than horizontal. Lateral aim is straight ahead, but must be no more than 150 mm either side of vertical.
If the beam is asymmetric, aim is measured at the left of the top edge. They must be 60 mm above vertical with an error of no more than 175 mm above to 50 mm below horizontal. They must also be aimed straight ahead but must be no more than 100 mm either side of vertical.