I think the most common reason for which snakes cross roads is the simplest of all, and it's the reason why all animals, included humans, must cross roads during the course of their life (if they live where roads are present): because roads cut their territory and must be crossed by individuals, if they want or need to continue in a determinate direction.
I have the sensation that, when a snake reaches a road, it has the feeling of something "strange" and new: I have for instance spotted Malpolon wait at the edge of a road with elevated head to control what was happening on that flat surface devoid of rocks or vegetation.
And snakes cannot certain understand and think: "Oh, my God, it's a road. Danger: I must crawl back now!" They continue in their direction and cross the road, often very fast. I've seen many species crossing the roads fast during the day: Hierophis viridiflavus, Malpolon sp., Elaphe quatuorlineata, Natrix natrix, Zamenis longissimus, Zamenis situla, Ptyas mucosa, Zaocys korros and probably others.
And I've also seen species crossing roads in a fast way during nightime: Vipera aspis, Vipera ammodytes, Coronella girondica (fast as it can be) and probably others.
And, on my experience, if a snake sees a vehicle approaching when it's beginning to cross a road, it normally reverses direction at once and disappears from where it came from.
But then we all know that many specimens of many species can cross the roads very slowly or even stay over them, in a more or less "elongated" or "en accordeon" way. But also in this case nothing strange happens.
Snakes rest on a road exactly in the same way as they can rest over other natural surfaces: over the leaf litter of a wood, over a field, over a stone field or over rocks.
I've seen snakes resting on roads during daytime (Z.situla and longissimus, E.quatuorlineata, Hierophis sp., Malpolon sp., Vipera sp., Coronella sp., Natrix tessellata, Natrix maura and others) and during nightime (Coronella girondica, Z.longissimus and situla, Telescopus fallax, Trimeresurus sp., Boiga sp.).
I would say that, mainly at our latitudes, resting in an elongated posture over a road can be a way of thermoregulating. Probably the sensation, in the early morning hours or in the evening hours, of the warm and smooth asphalt under the belly is not a bad sensation for a snake, which is used to more uncomfortable materials and type of belly contacts.
In this way we can explain why many species, especially the calmer ones (Zamenis sp.), tend to rest in "en accordeon" way over the asphalt: while slowly crossing a road, they feel very comfortable over the asphalt! And doing this they rely "naively" upon their camouflage as they would do (and I've seen they do!) in a wood or in a field, remaining totally motionless when a vehicle approaches them.
And then, over the roads, snakes can do all the activities they normally do "in the woods". I've seen e.g. snakes mating and/or fighting on the road (H.viridiflavus) and snakes hunting frogs on the road (Natrix natrix and Bungarus andamanensis).
Paradoxically in some tropical rainforest regions, which are clearly full of snake species and individuals, it's sometimes very difficult to see snakes crossing roads during the day. This is probably due to the very high temperatures and the absence of the need to termoregulate over an exposed surface. So, when I travelled by car for many chilometers in Borneo to reach the Danum Valley over a dirty road, I couldn't see any snake. And in the asphalted and rather trafficked roads of Borneo I met only one or two roadkilled snakes in hundreds of chilometers.
Same disappointing experience I had in India, where travelling by car during nighttime for many chilometers among perfect snake habitats did not give any positive result.
But: during heavy rainfalls, the roads in the tropics can become a paradise for snake fans during nighttime. We experienced this fact many nights in the Andamans, where, during rainfalls, we found really many snakes, included Bungarus which were feeding upon (probably dead) frogs over the roads.
Why so many snakes on the roads during night rainfalls?
Probably (I think) for more than a reason: presence of many preys on the roads (frogs); flooding of normal sites and dens of snakes; decrease of the otherwise too hot temperatures.
So: this is my experience and my thoughts about the problem "snakes and roads"