There was an in depth Thread on this subject some months ago. The general consensus of those who had researched the subject, was that roads were not generally used on a set piece battlefield because of their bottleneck effect. Off the battlefield maneuvering across a region, maybe one column of an army would be on a road (if it was straight enough and wide enough) but the other columns would be flanking and marching through fields etc.
You have to picture in your mind 70,000 to 100,000 soldiers, their arty, baggage, ammo etc... marching along a road, the rear of the army would be some miles back, the marching army would not be in any type of battle ready formation if surprised. The French Corps structure, which generally meant each Corp was a small army, generally advanced across country in as straighter line as possible to it's destination. Also the broader the front of advance the faster the deployment to battle positions. Napoleon was famous for his Army's ability to out march and out maneuver his opponent and he couldn't do this, constricted by the roads of the time.
This said I did not personally do the research, and I know someone will point out an exception.