There is a big difference between the X15 and X9/X10.
The X15 is on and off the A1 and, for most of the route, it is a standard single carriage way like most roads (minus the Dual Carriageways and Motorways). I am aware that some of the X15 route is on a dual carriage way (like when it first joins the A1 heading north at Gosforth). However, all of the A19 covered by the X9 and X10 is dual carriage way and doesn't keep exiting at every single possible point (not saying the X15 does that because i know for a fact it doesnt). That could be something to do with it.
Go North East: Latest News & Discussion - August 2015