Not in the way you are using it.
"Small is the opposite of big" is saying that the two words mean opposing things. This is okay, because "of" indicates a difference in essence.
"Small is opposite to big", however, indicates an opposition in physical position. It suggests that "X" is in the opposite position compared to "Y". The "to" in this phrase indicates a physical opposition, where the words "small" and "big" do not have physical entities to represent them, and it would only make sense if you were putting the words physically in a list, and saying they were opposite to each other on that list.
It depends on how exactly the state of the brightness of the lights needs to match the pre-dimmed state, and what level the brighter setting is. I.e. if you just want the lights set brighter than they currently are, or to be set to the highest brightness setting, you can be less specific in your instruction.
My choice for a less specific instruction would be (as mentioned in several comments):
turn up the lights / turn the lights up - if the lights were not previously set lower/if the lights being previously set lower is irrelevant
turn the lights back up - if the lights were previously dimmed
The second of these gives the implication of returning to a previous state, but doesn't explicitly say that it must be to exactly the previous state.
Turn is especially suitable if it is a rotating dimmer switch
If there is a specific setting for the lights to be returned to (e.g. going from 1/4 brightness to 1/2 brightness, or changing to half of the lights at full brightness, half of the lights at 1/2 brightness) then you need to be more specific about it. Words like reset, restore, return or redo may be appropriate.
Note, however, that these terms do not specify that the lights are to be brightened.
Also note that just saying:
do the lights
may well be understood in the context. It can also be used to mean dim the lights; for example before a slideshow you could say "do the lights" to mean dim the lights and afterwards also say "do the lights" to mean make the lights bright again.
Best Answer
I've never heard or used "busy traffic" in this context. Rather, the roadway is what is busy with traffic. So I would use: "The traffic is usually heavy during rush hour," which would make the opposite "light traffic."