The top red line shows where the Roman road 'ought' to go, and if I am right, where it actually does go (apart from a slight deviation to the south in the Deptford and Greenwich area); the bottom line shows its present route.
I have found a method of finding Roman roads by using a form of 'field walking', that is, by checking for any unusual subsidence patterns that an object as massive as a Roman road might have generated.
This is not an entirely new method, it has long been recognised that in open countryside traces of Roman roads often remain, but it is generally assumed that it is not possible to find any traces in built-up urban areas. However, it would seem that even where areas were completely levelled during the urbanisation of London that under certain conditions further settling takes place leaving characteristic undulations, particularly on the kerbstones of pavements. Bizarrely the effect is strongest in the low walls often found around council estates.
I drew a pencil line on a map of the theoretical alignment of the continuation of Watling Street and followed this as best I could to see if any traces were still remaining. Initially I did not expect to find much because South London is now heavily built-up. To my astonishment I found this strong and unusual subsidence pattern on a council estate just to the north of Elephant and Castle almost exactly where my pencil line had predicted it to be, and I knew I was on to something.
Falmouth Road, Rockingham Estate, SE1
In fact, these indications of what is almost certainly Watling Street occur along a considerable length of where the Roman road 'ought' to be, that is, following the straight alignment.
I found this particularly strong subsidence pattern over a mile to the east of Elephant and Castle in the oddly named Galleywall Road (this name has very obvious Roman connotations - a galley is a type of Roman ship). It is an almost identical feature to the Falmouth road one and probably represents the road having been 'robbed out', that is, its materials have been dug out for other building purposes.
Galleywall Road, Bermondsey, SE16
The most curious thing about this original route is that after passing through Greenwich and Deptford it continues straight through South London to cross the Thames at Westminster. After this it almost certainly joined up with the North London Watling Street, the present Edgware road.
If my theory is correct then the original Watling Street was built to cross the existing crossing at Westminster (almost certainly a ford). It was never intended to cross at the London Bridge crossing as is usually assumed, and this raises questions as to why the Romans chose to make London their city and not Westminster. (see pre-Roman London and)
This is a transcription (merely changing the strange long 'S's with modern ones) of a document I found at the Guildhall Library in London (it was too old to photocopy) by checking a reference that the road had been seen in South London in the eighteenth century. It is part of a description of the route Watling Street took when it was still visible, before much of South London was built upon. This was written by the famous antiquarian William Stukeley (best known for his research on Stonehenge). He has followed Watling Street down from Warwickshire and this is how he describes the missing section.
William Stukeley 1722
Hence the road goes through Edgworth; and so at Paddington, by Tyburn, it crosses the other Roman road, called now Oxford-street, which was originally continued to Old-street, going north of London one way; the other way it proceeds by the back side of Kensington and through an unfrequented path, till it falls into the present Great road to Brentford, Stanes, &c. and it is a Roman road all the way, going pretty nearly east and west: therefore our Watling-street must cross it with an oblique angle; and by observation I found it to be about forty-five degrees. Higden takes notice that Watling-street ran to the west of Westminster over the Thames, so through the middle of Kent: from Tyburn I judge it goes over part of Hyde park,* and by May-fair, through St. Jamesís park to the street by old Palace-yard called Wool-stable, to the Thames, here has been an old gate; one arch left but not Roman. On the opposite side of the river is Stan-gate ferry, which is the continuation of this street to Canterbury, and so to the three famous sea-ports, Rupiudae, Dubris and Lemanis. This Oxford road was originally carried north of London, in order to pass into Essex, because London then was not considerable; but in a little time became well nigh lost; and Holborn was struck out from it, as conducting travellers thither, directly entering the city at Newgate, originally called Chamberlainís gate, and so to London-stone, the lapis millitaris from which distances are recorded: and hence the reason why the name of Watling-street is still preserved in the city, though the real Watling-street goes through no part of it, but through Southwark; or if we please, we may call this a vicinal branch of the Watling street.
According to method I should speak of Londinium here: but because the great deal that may be said thereupon will make a discourse by itself, we contend ourselves at present with giving a plan of it, as we suppose it might appear in the times of the Romans; and so continuing our tour into Kent, will finish the whole continuation of Watling street with what few memoirs I could pick up at that time.
As Old-street went on the north of London, so the proper Watling-street we have been upon, since High-cross in Warwickshire, went on the south; from Stan-gate ferry across St. Georgeís fields, so south of the Lock hospital to Deptford and Black heath: a small portion of the ancient way pointing to Westminster Abbey is now the common road on this side the nearest turnpike; but the continuation of it is quite lost since the bridge was made, and all roads meet at that centre as so many radii when London became considerable, the ferry over-against it, from being better attended, rendered that at Stan-gate almost useless; so passengers went through the city by Canon-street, Watling-street, and Holborn; hence so little appears of it between Tyburn and the Lock hospital; and probably its materials were long since wholly dug away to mend the highways. Upon this way in Southwark many Roman antiquities have been found, particularly a Janus of stone, in possession of Dr. Woodward: but our business shall be to prosecute the end of the second journey and the whole third and fourth of Antininus.
From Shooters hill the direction of the road, very plain both ways; a mile westward from the bottom of the hill you find vestiges of it just upon the common: some part of the agger is left, made of gravel near at hand: from the top of Shooters hill you see it butts upon Westminster abbey, where it passes the Thames; and this demonstrates its original direction, and that it was begun from the east; for the turn of the river at Greenwich intercepts it, though not observed in maps: so the way is forced to deflect a little southward there, and then recovers its point: beyond that hill it is very straight as far as the Ken reaches. On Black heath a vast tumulus, now used as a butt for archers, hereabouts in great request till Henry the VIIIthís time and hence the name of Shooters hill.
*A brass Roman lar dug up about Grosvenor square (in possession of Mr Benpre Bell) near where the Roman road ran the Watling street.
Lock Hospital was closed in 1760. The site is now covered by the junction of Tabard Street and Great Dover Street.
Vicinal: neighbouring, adjacent, local.
I have also been trying to find out where it went north of the Thames, this has always been a mystery as all traces of it are supposed to have vanished at Marble Arch, where the Edgware Road meets the roman Oxford Street. However by extending the alignment