At the height of its power in the 2nd century, the imperial Roman army was a formidable force. It had conquered much of the known world, from northern Scotland to the banks of the Euphrates in Mesopotamia (modern Iraq). The army could build almost anything - bridges, tunnels, forts, canals, marketplaces and bath houses - but its greatest achievement was a network of roads over more than
55,000 miles (88,000 kms) of the Roman Empire.
A smooth surface for marching feet
The first true Roman road was begun in 312 BC to link Rome with Capua in the south; it was later extended to Brindisi in the east. The route, known as the Appian Way, set new standards in road building by providing a dry, relatively smooth surface that was both firm underfoot and well drained.
Moreover, in common with almost all Roman roads, the Way was remarkably straight, providing troops on foot or on horseback with the most direct route possible.
The course of the road was plotted by surveyors, using just two instruments: a portable sundial, to determine direction, and a groma - a horizontally held wooden cross with four vertical plumb lines - used for gauging right angles and marking out land.
Once this was done, the roads were meticulously constructed, using layers of stones through which water could drain, and with a porous core that would solidify over years of continuous use.
Major roads started as a base of compacted earth at a depth of 3 ft (1 m), with a recommended width of 18 ft (5.5 m). This was followed by a shallow layer of small stones, and then a deeper one of broken stones, tile chips and concrete. This was topped with slag, lime, chalk or tiles and then finished with paving stones. Paving slabs ran along the sides of the roads which were occasionally divided by a central gutter.
The Roman Empire fell in the 5th century AD, but its remarkable roads survived in reasonable repair for another 1,000 years or more: some remained the finest roads in Europe well into the 18th century.