Most light bars aren't exactly road legal, so if there is other traffic on the road, you should shut it off. The light patterns (though fairly well focused) would still get up in on-coming traffic's eyes like no tomorrow. I run a Rigid Industries SR2 20" combo light on my truck. Great for driving on highways at night in the middle of nowhere. Costed me just north of 700 for my unit, but it puts out more light than many cheap 40" double-decker units. The two brands I really was torn between for me are Baja Designs and Rigid Industries. Looks like they put a ton of effort into their optics and their products show it. It's just that there were easier mounting options for my vehicle with Rigid than with Baja at the time.
Rigid also makes DOT-compliant driver and flood beam LED lights as well. The flood lights, you can leave on with on-coming traffic as it really just lights up the periferals. The driving lights provide for good visibility forward, but not as much as their more focused hyper-spot or spectre lenses (which is what my unit is a combo of).