The B645 was originally part of the A45 - and indeed came to be the least-used section as unlike the remainder it was not upgraded; traffic was directed along the A604 further north. With the completion of the A14 in the 1990s the eastern section of the A45 was renumbered as part of the A14 whilst the western section was diverted to meet it. What remained became the B645. Travelling the road today will make it clear why the road did not retain Class I status.
The road starts at a roundabout on the A45 Rushden bypass at the point where that road leaves its original route. The B645 heads east along Northampton Road to reach Higham Ferrers and a T-junction on the A5028 (pre-bypass A6). There's a multiplex north for about half a mile through the centre of town and then the B645 regains its number by turning right at a mini-roundabout. It's not long before the present line of the A6 is crossed at a roundabout and we enter open country.
Water Tower by Three Shire House
After passing a golf course we enter the first village en route, Chelveston, and zigzag through the village streets, meeting the B663 at one of the bends. We continue to wind across the fields and climb to cross a ridge before following the Northamptonshire/Bedfordshire county boundary for a couple of miles, skirting the southern edge of Hargrave as we do so. At another bend we pass Three Shire House, where Northamptonshire (on the left) is replaced by Huntingdonshire. Another mile or so further on we leave the county boundary behind and pass a farm in Bedfordshire before entering Huntingdonshire properly.
After skirting the southern edge of Tilbrook we enter Kimbolton, the largest place on the road except at the ends. The B660 comes in from the left on the green at the near side of the village and then we multiplex through the centre, with the B645 dominant. Kimbolton is one of those places crying out for a bypass and is perhaps the main reason why the A45 no longer goes this way.
The entrance to the village is along a straight road but there is then a sharp zigzag right then left around the churchyard onto the High Street. This road is wide enough for cars to park end-on and still leave room for traffic; unfortunately it doesn't last. We head towards a gateway into the castle grounds (now a school) then bear left to avoid it and then turn very sharply left just afterwards - signs advise drivers to exercise "extreme caution". On reaching the River Kym there's another sharp right-hand bend (not quite as bad this time) and we continue on past the school grounds. The B660 leaves us at the end of the estate, after which we cross the River Kym and enter Stoneley.
Back in open country the road crosses a low hill before descending back into the river valley to reach Great Staunton. The B661 (from Grafham Water) comes in from the left, after which we go through the centre of the village, with yet another sharp left-hand bend. The Kym is crossed once more after which we bypass Hail Weston (as the A45 always did) and then continue on to meet the A1 St Neots bypass in Eaton Ford. There's a GSJ with that road and we morph into the B1048 somewhere here.