The A605 road is a main road in the English counties of Northamptonshire and Cambridgeshire.
The A605 strikes north from junction 13 of the trunk A14 road through the eastern parts of Thrapston, skirts the village of Thorpe Waterville, bypasses Oundle to its east, crosses the River Nene, reaches the eastern limit of the A427 at a roundabout, skirts to the west of Eaglethorpe, crosses into Cambridgeshire near Elton Hall, to reach the A1(M)'s junction 17 whereupon it overlaps the A1 road. It then continues north east through Whittlesey before terminating at a junction with the A141 between March, Cambridgeshire and Guyhirn. Basically, except for a short section at Oundle, its route follows the south bank of the River Nene.
The road used to start at the A6 just north of Higham Ferrers and run to Thrapston and Peterborough, following the line of the Roman Road between Irchester and Durobrivae (Wansford). The road split in two just before the A6 with one branch joining the A6 by the Irthlingborough Viaduct and the other at the top of the hill as the A6 enters Higham Ferrers. The later branch survives as a local access but the former has been returned to field.
In the early 1980s, development of the route from Northampton to Thrapston saw the building of a new section of the A605 mostly on line between Chown's Mill roundabout and the Stanwick roundabout. This was later dualled. A year later the Stanwick and Raunds Bypass was opened with land for dualling which, as of 2009, has not been carried out. In the mid-1980s, the Ringstead and Denford Bypass was constructed from the Raunds roundabout to a grade separated interchange at Thrapston with the A14. With the completion of the route, the Chowns Mill roundabout to Thrapston roundabout section of the A605 was renumbered the A45 and the A45 to the east of Higham Ferrers was renumbered A4500. Thrapston is now the terminus of the A45 and the start of the A605.
Just before the A14 was built, a short section of the A605 was built bypassing Thrapston to the east to the A604 (as it was then known). This section began approximately 0.25 miles (400 m) northeast of Thrapston and terminated at a new roundabout on the A604, now merely an access to Thrapston industrial estate. The Thrapston Eastern Bypass was opened a few years later running northeast from this roundabout to a new junction with the old A605, just south of the Titchmarsh turn. Excavation of a Roman way station on the site of the junction proved disappointing, with no significant finds. The junction at the northeast end of Thrapston was converted to a roundabout around 2005. A bend was also eased between Titchmarsh turn and Thorpe Waterville. Around 2000 the A605 was widened between Thorpe Waterville and a new Lilford roundabout was constructed. An original milestone was unearthed and set on the side of the north east exit.
The road was improved from Barnwell turn to a new roundabout northeast of Oundle as the Oundle Bypass. This used the alignment of the former BR Northampton to Peterborough railway and crosses the River Nene twice in quick succession. The road was opened on 12 December 1985 by Princess Alice, Duchess of Gloucester and is commemorated by a plinth and brass plaque at the roundabout. A minor alignment change took place beyond here through the hamlet of Elmington. Beyond Elmington a curve was eased as the road climbed out of the valley and a new straightened section was constructed to a new roundabout for Warmington. The old road enters the garage just south of the roundabout. The Warmington Bypass was one of the last improvements on the A605 and was completed in the late 1990s, including a section to the county boundary. The old road south of Warmington roundabout is now a garage access and the old road into Warmington can be seen on the right a short distance after the roundabout.
The Elton Bypass takes the A605 onto a new more southerly route to join the A1(M) at its northern terminus at Junction 17. This route then overlaps the A1 for 1 mile (1.6 km) to Alwalton where it regains its original route, heading northeast through Orton towards Peterborough. The bypassed section from Elton to Orton suffered from poor alignment and is now the B671 and a C road. The Elton Bypass climbs two hills and benefits from crawler lanes on both sections from both east and west.
East of the A1 the route approaching Peterborough has, in effect, become a local distributor as traffic for the A605 onward to Whittlesey/March etc. will use the parallel A1139 Peterborough Southern Bypass from Junction 17 on the A1(M). There are four new roundabouts between Alwalton and Orton. A diversion cuts a sharp corner between the third and fourth roundabouts. There is a limited access junction onto the A1260 Peterborough Western Bypass. The road passes under the East Coast Main Line and has a roundabout junction with the A15 immediately south of Peterborough city's Nene Bridge. The road turns south and overlaps the A15 for a short way before originally diverging to the left and passing under the Peterborough Southern Bypass (the A1139) at Stanground. Finally, there is junction with the A1139 access road and the A605 turns east towards Whittlesey on what was once an old toll road. In December 2009 a Stanground Bypass (mostly dual carriageway) was opened on the eastern approaches to Peterborough. This actually lengthens the route of the A605, although those using the A605 and Peterborough Bypass towards Whittlesey will find it shorter.
At no point in its route does the A605 meet another A-road with a 6-series number. This apparent isolation is the result of it being truncated at its southern end, and the renumbering of the A604 (crossing at Thrapston) to the A14. It used to leave the A6 road at a junction north of Higham Ferrers but this stretch has been renumbered as the A45 road that terminates at the same A14 junction as the A605 now starts from. It is a matter of speculation why the whole of the A605 was not similarly renumbered.