A car is not the best way to get around Oxford, there are far too many traffic jams. The quickest way to proceed is by bicycle or even better by motor scooter. There is no better illustration of this than the time I was waiting in a queue of traffic at the foot of Headington Hill, Oxford, for a red light to change. Suddenly the lights went green, handbrakes hit the floor, feet tickled the gas pedal, but the car at the front didn't shift an inch. As the seconds ticked by it was clear that the driver was not about to move despite some furious horn-blowing from the cars behind. What could be the hold up? An intractably stalled engine? A cardiac arrest? I decided not to wait. I manouevred my scooter to the front of the line and peered through the driver's window ... only to find a middle-aged lady completely absorbed in a newspaper Sudoku puzzle! Technology had found its limit and a traffic cop might have been far more efficient at getting the traffic moving.

It turns out that the first traffic light system still needed a traffic cop for its operation, and although thankful to be relieved of arm-waving duty, he still had to direct the traffic. The apparatus went into use in December 1868 at the junction of Great George Street and Bridge Street in the London borough of Westminster and was based on the railway signalling system. Credit went to the Nottingham engineer John P. Knight who had the idea of employing lever-operated semaphore arms during daylight hours while at night, red and green gas lamps mounted on the ends of the arms were turned towards the appropriate line of traffic to convey instructions. Unfortunately, less than a month after its installation, a gas leak caused an explosion which fatally burnt the police officer on lever-pulling duty.

The first electrical system was invented in 1912, by Lester Wire (I hope he never introduced himself as Mister Wire) of Salt Lake City, Utah, but the actual operation of an electrically powered unit had to wait until August 1914 when red and green lights were sited in Cleveland, Ohio. The three-colour light system which we are familiar with today first appeared in Detroit, Michigan six years later ... which is just as well because without it there would have been no puzzle this week!

The latest offering demands a particular way of looking at traffic lights.

A woman driving through town chances upon three traffic lights illuminated in the order shown. This prompts her to look in her rear-view mirror to confirm that she has forgotten to buy something. What is it? (3 letters)

The solution to The Concealed Car Puzzle is found by opening Door D. The reason that the doors are made of glass is so that when Door D is fully opened and superimposed onto Door C the representation of a car appears (as shown) using features from both doors. This accounts for why the doors are situated close together.