Its a person digging. Its based on the UK roadsign by Margaret Calvert (who inspired roadsigns across the world – see the German or Australian ones for the same).
She is on record as saying she wished she’d made the sign more shovel-like.
She told Frieze: “I now regret that I didn’t put a corner of a spade
on the ‘men at work’ sign, it would have stopped all the jokes about a
man struggling to put up an umbrella!”
I guess its a salutary lesson about design, if its not 100% clear, its going to be confusing for all of future history!
Here’s the US Department of Transportation’s official description from the 2009 MUTCD:
W21-1 is shown as a diamond-shaped sign with a symbol of a right-facing person with a shovel digging.
Credit:stackoverflow.com‘