I've written about this before but I can't find the god damned post for the life of me. In either case, yes, the front was mostly empty aside from strategic locations for exactly the reason you mention here, it is neither possible nor practical to cover the entire front. First of all, you just don't have enough men for that, second, there is no reason to cover every inch of the front.

Generally speaking you want to cover strategically important ground. That can be roads as you mention but it can also be choke points like mountain passes, important high ground etc. The rest of the front is generally patrolled by mobile units to ensure that no major enemy forces are around but generally speaking armies cannot move as freely as we tend to think. If you're not familiar with large scale logistics you can easily get the impression that any area is equally viable as a route of attack but in reality that isn't the case. There is a reason why a lot of areas have been historically hotly contested and very often that reason is logistics. A large modern army consumes enormous amounts of supplies every day and if it out-runs its own supply train it will very quickly be forced to stop.

If we look at the eastern front of WW2 in particular you'll find large concentrations of armies in some areas while others were held by under-strength divisions that could have assigned frontlines that were hundreds of kilometers long. So how did they cover such a massive area? They didn't. Most of the division would have been concentrated to one or a few key areas with a few mobile units patrolling the front and a few outposts in important areas such as crossroads, generally supported by reconnaissance aircraft and generally that's enough. The idea for these units is not to stop an enemy offensive, just to report it. Once you know the enemy is coming you know approximately which way they'll come because there are only so many potential paths it can take. At this point you move your main force into a defensive position of your choice and attempt to stop the enemy.

I actually just answered a similar question this past week. I wrote about the topic of division frontages a while back, and in this more recent post the OP was curious about how the front was stitched together at the operational level. In both posts I reference US Army practices during the Second World War.

The post regarding divisional frontages can be found here: https://www.reddit.com/r/AskHistorians/comments/lcpi2l/how_many_miles_of_front_would_an_infantry/?utm_source=share&utm_medium=ios_app&utm_name=iossmf

The more recent post, in which I linked the above answer before continuing to address battlefield coverage between corps and armies, can be found here: https://www.reddit.com/r/AskHistorians/comments/mph4sp/what_did_the_frontline_in_ww2_look_like_was_it/?utm_source=share&utm_medium=ios_app&utm_name=iossmf

u/Superplaner’s answer is also spot on. Again reaching into the well of the US Army’s experience during the Second World War, if you look at the fall 1944 campaign along the German border you see that two field armies, the First US and Third US, placed the bulk of their forces around the cities of Aachen and Metz respectively. This is because, to reference u/Superplaner’s comment regarding the same ground being fought over, the terrain in those areas are conducive to traversal by large armies, resulting in their usage as traditional gateways between France and Germany. Meanwhile, at the same time a single corps of 3 divisions of First Army covered a front of nearly 100 miles in the Ardennes, which is sandwiched between Aachen and Metz and characterized by terrain that is not nearly as conducive to the movement of large mechanized armies as that on its flanks.