Longest distance a road uses an abandoned railway alignment?

Curious to see if anyone knows anything longer.
A16 - 13 miles (11 Spalding - Boston) (2 Grimsby - Holton Le Clay)

The SABRE wiki says the A16 Spalding to Boston stretch is "probably" the longest railway-to-new-road conversion in the country at 12 miles, so I think you've already won with that suggestion!

Nowhere near your example on distance, but the Fareham - Gosport Bus only section of the E routes uses part of the long gone rail line between the two towns.

Great topic @SomeTrainboy

Nowhere near as long as the A16's re-used alignment, but:
The Imerys private haul road running from Par Harbour to Fowey Docks (converted from the railway line following its closure in 1968) is approximately 4.25 miles (6.8 km).

A3088 follows a railway line for most of its route but is only 5.9 miles

Just how far does the A66 now follow a former railway alignment up in the Lake District?

The A470(T) has quite a few bits of railway heritage underneath it, but is of course a long road extending from Cardiff to Llandudno:

At Tongwynlais (north of Cardiff), it picks up the formation of the former Cardiff Railway line from Cardiff to Pontypridd which was never used for the intended coal traffic as the Taff Vale would not allow use of the junction at Pontypridd. The road uses the old trackbed in sections and at the same time uses the route of the old Glamorganshire Canal and there probably places where it uses both.
From Quaker's Yard (north of the Aberdare junction) it picks up the route of the Quaker's Yard and Merthyr line (Great Western and Rhymney joint), again sometimes deviating significantly but this stretch is several miles long. The line closed by 1950 as a through route due to subsidence damage to the viaduct at Quaker's Yard, but had been a duplicate line since the grouping to the Taff Vale route on the other side of the valley.
Not long after it leaves the previous route it then joins the LNWR/Brecon and Merthyr Joint line from Merthyr to Morlais Junction where the B & M line to Brecon split from the LNWR line to Abergavenny. It doesn't follow the whole route so we still have the splendid masonry viaduct at Cefn Coed.
The road takes the route of the railway through Dolgellau, mainly through where the station was. This was the GWR route from Llangollen to Barmouth Junction (Morfa Mawddach), part of which is a heritage line today.
There is just a short stretch north of Trawsfynydd where the road uses the old trackbed, but mainly the two ran side-by-side.
As a final flourish, on entry into Llandudno it uses the same street (Mostyn Street) as traversed by the Llandudno & Colwyn Bay Electric Railway. However, as a street-running tramway that one probably doesn't count! It started on a former tram route too, the route to Pier Head in Cardiff.

Mcr Warrior said:

Just how far does the A66 now follow a former railway alignment up in the Lake District?

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Based on oldrailwaymaps, not much really..
There's bits and pieces about, around Bowes, Appleby and Kirkby Thore.
The A1(M) does follow a route for 5.6 miles north from Jnc 56 (Barton) to just south of Jnc 58.

BeeBus said:

The A470(T) has quite a few bits of railway heritage underneath it, but is of course a long road extending from Cardiff to Llandudno:

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Also through Llanidloes on its by-pass.

SomeTrainboy said:

Based on oldrailwaymaps, not much really..
There's bits and pieces about, around Bowes, Appleby and Kirkby Thore.
The A1(M) does follow a route for 5.6 miles north from Jnc 56 (Barton) to just south of Jnc 58.

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The A66 in the Lakes was built on the old railway alignment from just east of Cockermouth, along the edge of Bassenthwaite Lake, to just north of Braithwaite, a distance of some 8 to 9 miles. You can still see the station houses at Embleton and Bassenthwaite Lake next to the road.

D1537 said:

The A66 in the Lakes was built on the old railway alignment from just east of Cockermouth, along the edge of Bassenthwaite Lake, to just north of Braithwaite, a distance of some 8 to 9 miles. You can still see the station houses at Embleton and Bassenthwaite Lake next to the road.

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So it is! Sorry I didn't look that far west.

The A43 just west of Northampton is built on the alignment of the Northampton - Blisworth line.
Unremarkable in itself - it's not a great distance - but unusual in that as a railway it was being electrified - masts definitely up, not sure of spans. Definitely re-signalled with MAS.

Jüst possible there's a road on the alignment of a third or 3rd & 4th rail electrified line.

The M90 between Cowdenbeath and Bridge of Earn? Not sure how long it is though.

D6130 said:

The M90 between Cowdenbeath and Bridge of Earn? Not sure how long it is though.

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I think only about 3 miles (perhaps less) is on the alignment of the old Glenfarg route. Whenever I drive it, I wonder what might've been, if the railway line had been retained there.

Not sure if Cambridgeshire Guided Busway counts as being a road. Its section from Cambridge North to St Ives is 13 miles.

dmncf said:

Not sure if Cambridgeshire Guided Busway counts as being a road. Its section from Cambridge North to St Ives is 13 miles.

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and another 2 miles at the Trumpington end.

Another long Fenland example is the A17 between Sutton Bridge and the outskirts of Kings Lynn, about 8-9 miles.

Although possibly not continuous, am I mistaken in thinking that the A465 Head of the Valleys Road follows the formation of the ex LNWR Merthyr, Tredegar & Abergavenny for much of the way?

What would the mileage be of the Cheshire Lines Railway be where the Southport Coast Road be from the Lord St terminus to approx where the Haskane joined with the L+Y line to Southport?

Another two and a half miles between Barnwell and Oundle where the A605 follows the alignment of the Peterborough/Northampton railway.

North Devon Link Road (A361) partially uses the course of the Taunton to Barnstaple line, including reusing the pillars of Filleigh Viaduct.

Merle Haggard said:

Although possibly not continuous, am I mistaken in thinking that the A465 Head of the Valleys Road follows the formation of the ex LNWR Merthyr, Tredegar & Abergavenny for much of the way?

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The road certainly runs on a similar route, however only a fairly short section actually uses the alignment. From the edge of Abergavenny to Brynmawr the route is a path.

I was recently looking at this line with a view to exploring it later this year, as a rough idea as to how much of the route is actually claimed by the A465 I think only two station sites are under the A465.

Part of the A34 sits on the railway line north of Whitchurch (Hampshire)

The A385 between Newbiggin and Tebay runs for 7.5 miles along the trackbed of the former South Durham and Lancashire Union Railway, aka the Stainmore Line.

JonathanH said:

Part of the A34 sits on the railway line north of Whitchurch (Hampshire)

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Also through Kingsworthy (just north of Winchester) the A34 bypass is on the railway alignment. Also near Lichfield, and quite a big chunk of the Newbury bypass at its south end. I think added together there’s probably quite a few miles of A34 on the railway‘s route, possibly high single figures?