The problem, it seems, was that locking a parked car wasn’t always entirely socially acceptable or even permitted, because parking areas were often filled chaotically.
This 1927 Autocar article about golfing gives a good outline: “The man whose car is imprisoned by your own probably finishes his 19th hole when you are ignominiously engaged in excavating the bunker guarding the 11th green.
“With strange oaths he enters your car, takes off your brake and gets the rest of his foursome to push your car out of his way. They all cry ‘whoa!’ as the car slides up to the wall of the yard, the pawl of your brake catches on the quadrant, he struggles with it ineffectually, and either your tail lamp got concertinaed, or your rear panel is stove in.
“If you are a canny fellow with a saloon, which is carefully Yale-locked before parking, he will muster a battalion of caddies, and push it on locked wheels.”
The police expected that cars would be left unlocked so that they could be moved in an emergency, and this was made a legal requirement in London in January 1925.
Defending the law in 1929, by which time three people had been prosecuted under it, the Tory transport minister said: “Parking on the public highway is an indulgence and local authorities, having spent large sums of money in widening the streets, now consider that these should be used more for traffic than for parking places.” It wasn’t just a quirk of the capital, as shown by this 1929 news story: “The Leeds Watch Committee view with disfavour the increasingly common practice of leaving the doors of cars locked whilst they are standing in public parking places.”
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