OS 1:50000 v 1:25000

1:25000 OS maps have one major advantage over the 1:50000 series, and that's the way they show the difference between 'roads generally more than 4m wide' and 'roads generally less than 4m wide'.

The 1:25000 maps show <4m roads as yellow, and >4m as yellow-orange, with the latter being a distinct colour from the orange used for B roads.

The 1:50000 maps use the same orange for B roads, but show <4m and >4m both as yellow, with the latter having wider lines. It's not always easy to tell which width a yellow road is intended to be.

You might wonder why the 4m distinction is important: basically on a >4m road you can expect to pass a car coming the other way quite easily. On a <4m you can expect to spend your journey keeping mental track of how far back the last passing place was.

I personally never plan a journey using a <4m road, unless it's a very short, very useful connection between wider roads.