Ancient roads would follow the path of least resistance, and would simply be the trade path between one village and the next. if there was a paved path or a river crossing it would just be whatever the locals were using to get to market, or their farms, etc.
The Romans were among the first to build roads with a purpose, between Paris and the next large settlement on the way back to Rome. Whereas previously you would be zig zagging all across the countryside. Think of it as taking the highway versus using all the back streets to go from one side of america to the other.
There are also specific engineering things that made roman roads so useful. Romans would divert streams, use culverts (pipes under the road for stormwater), make the roads flat with proper crossfall to ensure drainage, etc. These things are not amazing today, and roman roads would not deal well with the traffic of modern times, but they allowed to romans to police a huge area and support a large empire with a comparatively small army.
APRIL FOOLS
This post was part of the 2014 AskHistorians April Fools joke. While the information about the construction of the road is accurate, roman concrete was not made from human bones nor were slaves used to truck in sand across the Alps. Likewise, to my knowledge, modern grave stones are in no way inspired by Roman paving stones
Roman and American road systems hail from much the same rational - namely the necessity of moving military troops around a vast land empire. For their time the Roman roads were of surpassing quality - far and away better than the American equivalents in the 20th century.
Roman roads are still in use today. Ponder that; more than 2000 years after they were laid down they are still in use. Of course, without the modern tools we use today the construction methods seem somewhat grisly to modern eyes. The plan for the roman road was born on the Italian peninsula. It depended on materials found there and, when those materials were in short supply they were obtained through slavery and conquest.
The road bed started with a ditch into which was first laid a layer of sand. In areas where sand was hard to find slaves would carry it overland in large clay pots. Blaccadicus writes that the "columns stretched, single file, across the Alps from the Aegean shores bringing sand for Caesars' gallic road"
Atop the sand was a layer of crushed rock (or clay pots). Followed by a layer of gravel in a cement mortar and another layer of sand, gravel, and more cement.
Cement was the key component. It bound the road together from the flat surface rocks to the crushed rock underpinnings that sat on that bed of sand. Good cement was the lynchpin of the Roman empire and it depended on volcanic ash where it could be found and more macabre sources where it could not.
The general Marcus Vindictus described a process by which the bones of slain enemies could be fired with limestone to create a simple binding agent. Modern historians have noted that the battles of the ancient world would not produce enough dead to account for roman road construction, however, leading to speculation that the additional bone matter came from women and children put to the roman sword. Indeed one need look no further than Romania for evidence of this -- a modern enclave of romance language surrounded by slavic ones and bearing, even today, a name which echos its conquest: the original inhabitants were butchered by the legion and likely lie beneath the Via Pontica.
The final layer of paving stones served as a road surface and as the origin of the grave marker tradition in European culture. Even today gravestones evoke the flat paving stones used by the romans of the upright Milliarium milestones which dotted their roadways.
Source: "Mense quarto stulti iocus" [On the history of the Imperial road] by Titus Andronicus