2.As a substitute or alternative to; in place of: "walk instead of drive".
"Fog lamps instead of headlamps" implies the illumination of the fog lamps as an alternative to the illumination of headlamps. The fog lamps are on, the headlamps are off.
Now the "May
use fog lamps instead of headlamps" bit grants the operator the right to turn off the headlamps and use just the fog lamps for forward illumination, but only when atmospheric conditions make the use of the headlamps disadvantageous.
What the act does not do is restrict the use of the fog lamps in conjunction with
the headlamps to adverse weather conditions. It only restricts the use of fog lamps instead of
headlamps to adverse weather conditions.