The speed trap is as old as the automobile itself. In 1905, police in New York City used hollowed out trees to conceal themselves from passing motorists in a sophisticated trap. One officer would start a stopwatch and telephone ahead to a colleague down the road who would confirm when the car passed a known distance so the clock would be stopped. A chart provided an estimate of the driver's speed. One hundred and eight years later, the impulse to hide from the public remains just as strong.

Last week, police in Victoria, Australia revised speed camera policies to restore use of hidden speed trap cameras in the name of officer safety. Under previous rules, speed cameras were meant to be clearly visible so that motorists would slow down, achieving the stated goal of the automated enforcement program.

"Under no circumstances are camera vehicles, tripods or portable flash units (when used) to be disguised by signs, logos, breakdown of vehicle (e.g. boot open or spare wheel / jack visible etc), tree branches, lamp posts, rubbish bins or any other covert means," the 2010 speed camera rules state ( view old guidelines, 250k PDF).

The new policy allows the mobile road speed camera operator (MRSCO) to hide behind signs or trees in the name of hiding from angry motorists for safety ( view new guidelines, 50k PDF).

"Under no circumstances are MRSCO's to deliberately disguise a mobile road safety camera car or associated equipment whilst an operational session is being conducted," the new rules state. "This does not preclude a MRSCO from utilizing roadside vegetation or fixtures such as posts or signs in order to mitigate the risk of occupational health and safety incidents being initiated by other road users."

Another change in the rules overturned a ban on placing a speed camera within 300 yards of the bottom of a hill, a favorite speed trap location thanks to the added speed provided by gravity. The old rules discouraged the tactic as it encouraged drivers to ride their brakes, which can cause them to overheat. The new rules encourage a zero-tolerance rule on hills.

"There is no restriction from a technical, legislative or enforcement perspective on a mobile road safety camera being operated on a slope, hill or gradient," the new rules state. "All motorists have to comply with the relevant speed limit."

Fixed and mobile speed cameras in Victoria issued 1,106,773 tickets in the financial year ending June 2013.