be (as) plain as Dunstable road

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 road what I meant! These directions are plain as Dunstable road, 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 road.