Green’s Dictionary of Slang

(Aus.) a yellow speed bump placed in the centre of an intersection.

1934		T. Wood Cobbers 122: A circle in the middle of cross-roads, for example, round which all traffic changing direction must swing; a round yellow blob, known here as the Silent Cop, or the Poached Egg.
1948		R. Raven-Hart Canoe in Aus. 187: Motorists often ignore ‘Silent cops’ at corners, yellow domes with white surrounds, ‘poached eggs’.