In Milton Keynes, the road begins at a junction with theA509near Junction 14 of theM1 Motorwayand briefly follows H6 Childs Way westwards before turning south onto the V11 Tongwell Street, west again onto theH10 Bletcham Wayand once more to the south where itjoinswith theA5.It then separates again at the "Kelly's Kitchen"roundaboutheading west to bypass the southern flank ofBletchley.
At the end of this single-carriageway bypass, from a point just south of theWater Eaton estate,a new alignment has been built for the road to continue west on a short stretch of dual carriageway bypassing Linslade andStoke Hammond[1][2]51°55′N0°42′W/ 51.91°N 0.70°W/51.91; -0.70and to hug theWest Coast Main Linefor most of the way south towards Leighton Buzzard. The route is planned to tie in with a proposedA418Wingbypass. The new road features five new roundabouts, four over-bridges and three under-passes. The bypass was the focus on an anti-road protest.During the protest two women chained themselves to a digger. They were arrested and charged, but later acquitted.[3]
At a roundabout at the start of theA418just north ofAscott,the A4146 takes over the former route of theA505 roadaround the south of Leighton Buzzard. This stretch is partly a three-lane single carriageway and partly a wide two-lane single carriageway. On reaching a junction with the unclassified Leighton Road (Leighton Buzzard–Billing), the A4146 designation ends and the route continues eastwards as the A505.
