Road tax (I believe it's actually properly called Vehicle Excise Duty) has never been hypothecated for road repairs or maintenance.
Apparently, it was called road tax and used for road maintenance until 1937 (see below). It's been VED since then but still universally known as "road tax" over 80 years later. Much like the M.O.T. Test, which is still called that (even officially) despite the "Ministry of Transport" not existing since 1970.
Is there any such thing as 'road tax'? ~iw