These are two of the oldest roads/tracks in the country, both ancient when the Romans arrived. The Ridgeway is probably England's oldest road.
The Ridgeway is the older of the two, largely following the top of the generally north-west facing chalk escarpment across southern England from Dorset (near Lyme Regis), across Wiltshire, Berkshire (until 1974) (crossing the Thames at Goring/Streatley), Oxfordshire, Buckinghamshire, south Bedfordshire, north Hertfordshire and Cambridgeshire into south-west Norfolk. Neither end is clearly defined, and until the Enclosure Acts of the 18th century, neither was its route, because it consisted of a series of nearby tracks where people travelling along it used whatever was the easiest passage at the time. a very weather-dependent factor.
It passes very close to numerous Iron Age hill forts and other earthworks and ancient monuments (such as the Avebury stone circles, older, bigger and almost certainly more important than Stonehenge, and the Uffington White Horse) and undoubtedly served as the main communication route between them. Its age is unknown - it may even have been used in Stone Age times, providing a trading route for flint tools which were produced in Norfolk for use over a very wide area, although the earliest positive evidence of its use is in the Bronze Age. The reason it follows the top of the escarpment is simply that in those times the valley below was impassable forest and marshes.
The Icknield Way follows much the same route as the Ridgeway, but part way down the escarpment where it is less exposed to severe weather conditions and where water is readily available. The chalk hills are very porous, so water is scarce on the hilltops. Below the chalk is impervious clay, leading to numerous springs of very pure water where the clay/chalk interface meets the surface. Villages grew up along this spring line, and the Icknield Way passes through them. Since it was for centuries used as a main drovers' road, taking cattle and sheep from Wales and south west England to markets in the east (especially London), this supply of water was essential, while its elevation above the lowland meant it still avoided the marshes and forest below. The western end is again ill-defined, and the route, like that of the Ridgeway, was also slightly vague until the 18th century, but at its eastern end it joins the Peddars Way near Thetford, Norfolk. Thus the three roads, the Ridgeway, the Icknield Way and the Peddars Way provided a continuous route from the Dorset coast to that of north-west Norfolk (close to the recent discovery of a wooden henge submerged in the sea).
So far as I know there is just one point where the Ridgeway and the Icknield Way actually meet. This is at the Thames crossing at Goring, the "Goring Gap", where the river cuts through from the north side of the chalk hills to the south.
Being pre-Roman, both roads are far from straight, the Ridgeway tending to follow the contours and the Icknield Way generally following the spring line. At the time of the Enclosure Acts (1750-1800) both routes were officially defined by banks and thorn hedges to prevent herd animals in transit from straying onto the newly enclosed fields, and many of these boundaries remain today. They defined the width of the path as, in most parishes, 40 feet, but in some as much as 66 feet.
There are few references to the Ridgeway in the form of modern road names, for the simple reason that towns and villages could not survive at that altitude on the chalk for want of water. The Icknield Way, on the other hand, is a common road name in many towns and villages where parts of it are still used as roads today. Some towns and villages on the Icknield Way include Wroughton, Chiseldon, Liddington, Wanborough, Little Hinton, Bishopstone (all in Wiltshire), Ashbury, Compton Beauchamp, Kingstone Lisle, Sparsholt, Wantage, East and West Hendred, Harwell, West Hagbourne, Blewbury, Streatley, Goring, Ipsden, Ewelme, Brightwell Salome, Watlington, Chinnor, Princes Risborough (all now Oxfordshire, although those listed from Ashbury to and including Streatley were Berkshire until 1974), Aston Clinton, Tring, Ivinghoe (Buckinghamshire), Dunstable, Luton (Bedfordshire), Ickleford, Letchworth, Baldock and Royston (Hertfordshire). A section between Aston CLinton and Ivinghoe is in fact duplicated, a slightly more recent and lower altitude alternative passing through Marsworth instead of Tring.
Both roads are today part road (especially the Icknield Way), part bridleway (green lane), part footpath and in a few areas lost altogether as recognisable routes. Recently much of the Ridgeway has been opened up, with a few diversions from the original route (some along the Icknield Way), to re-create a long-distance footpath (officially so designated) from Dorset to Ivinghoe Beacon in Buckinghamshire. Another long-distance path now officially designated is along the Icknield Way from Ivinghoe Beacon to the Peddars Way in Norfolk (which has been similarly designated). With some gaps, most of the Icknield Way is tarmac road today, such as most of the B4507 (Ashbury to Wantage), B4009 (Ewelme to Aston Clinton), B488 & B489 (Aston Clinton to Dunstable) and A505 (Dunstable to just beyond Royston, with a break around the north of Luton town centre and Hitchin).