Interesting to read wikipedia, citing the 2 persepectives on this:
http://en.wikipedia.org/wiki/Automot...Rear_fog_lamps
Proponents of twin rear fog lamps say two lamps provide vehicle distance information not available from a single lamp. Proponents of the single rear fog lamp say dual rear fog lamps closely mimic the appearance of illuminated brake lamps (which are mandatorily installed in pairs), reducing the conspicuity of the brake lamps' message when the rear fogs are activated.
