For some time now, I've been intrigued by this sign on a local building site.
Apart from the familiar meaning of "slew", I know it as a noun. My old Oxford dictionary has the meaning of a swampy pond, which explains why it conveys unpleasant or disparaging associations to me: but it seems there's also an American usage whose meaning may come from an Irish word simply meaning "many" (evidence, no doubt, that I don't know my Erse from my elbow).
A verb whose present tense is "slew" is something new to me. On checking, I find it means to turn on the spot, or on the same axis, and by extension to swing round (and as a metaphor, to get someone drunk or to exhaust them with beating). So I suppose in this context it's - prosaically - simply a warning to the crane and machine drivers to be careful not to swing round and risk bashing the existing structure the sign's attached to.
But it also seems to me like a perfect substitute for that old political metaphor the "u-turn". Obviously, one has to be careful to avoid inadvertent libel, but, if the appropriate circumstances arose, even the Evening Standard might be tempted by a headline like BORIS: I SLEW ON BENDY-BUSES.