This was convenient, as I'd been meaning to take a photo of such a sign in Brighton, but since I'm not a tourist in Brighton, I rarely have my camera with me. So, it was great to see one while I had my camera at the ready on our mini holiday/vacation.
Needless to say (since I've posted a photo of it), this is not a sign you'd see in America. There, such a sign would probably have an unmodified slow or go slow.
In this context, dead is an adverb modifying slow. It makes me chuckle involuntarily for two reasons: (a) dead slow is not as idiomatic in AmE as in BrE and therefore the literal meaning occurs to me when I read it, and (b) in BrE adverbial dead is frequently a colloquialism, and therefore it seems a bit funny to see on a sign.
Since I get the literal meaning of dead slow when I read it, it strikes me as an oxymoron. If something's dead, it seems to me, it wouldn't move at all, so it couldn't be slow. But that "logic" is misplaced, since AmE, like BrE, uses dead as an adverb with other adjectives that indicate a glimmer (or more) of life--for example dead certain and dead tired. So, we could use dead with slow, but we tend not to.
If one hears a lot of colloquial BrE, one knows that dead can go with just about any adjective in certain informal registers. For example:
Dom looks dead sexy in eyeliner and black nail varnish (=AmE nail polish) [comment on blog.pinknews.co.uk]The OED, however, classes dead slow as a non-colloquial usage (going with dead calm and dead tired) rather than this all-purpose colloquial intensifier. At any rate, it all sounds dead British.
... I also watched "Sky High", which was dead good. [...] It's odd really, some of it is DEAD POSH, like the lobby and the millions of people tidying plates away at breakfast, and some of it ISN'T, like the mucky marks on the walls and the water dripping on your head in reception. [...] We then had a LOVELY bit of tapas (ooh, it was DEAD nice, roast potatoes and hot garlicy [sic] tomato sauce, ACE!) ... [a (orig. AmE) mother-lode of deadness in a description of a Singapore holiday from MJ Hibbett--I haven't bothered to mark all the other Briticisms in that]