be (as) plain as Dunstable highway

To be straightforward and easy to understand. The straight road of Watling Street connects the town of Dunstable, in Bedfordshire, to London. Primarily heard in UK. Why can't you understand what I'm trying to tell you? It's as plain as Dunstable highway what I meant! These directions are plain as Dunstable highway, so you shouldn't have any questions. The boss told me I have to drop the package off by 5 PM sharp because it must get delivered tomorrow—that part was plain as Dunstable highway.