Speed checks in the 1920s - Aretas
LOWESTOFT is going alter the heavy-vehicle driver who goes too fast. Nine of them were hauled before the county Bench and fined £2 10s. each. The speeds ranged from 18 to 23 m.p.h., and they were checked over a distance of a mile by the speedometer on a motor-cycle and sidecar driven alongside the offending vehicle.
The Auto 18 June 1925

Did they really mean alongside? Makes you wonder what the truck driver was doing>
Speed checks in the 1920s - doctorchris
Just shows that the authorities have always been prepared to put everyone at risk to achieve a prosecution. Plus ca change, plus c'est la meme chose.
Speed checks in the 1920s - Screwloose

23 mph!!! The mad fools! They could have died - speed kills, y'know....

I wonder how they checked the speedo on a 1920's motorbike? Quid custodes custodiet etc.

I'll bet the wagon-drivers were relieved when someone invented wing mirrors....
Speed checks in the 1920s - tack
Funnily enough, I have been researching my family history in the USA by going through newspaper archives from early 1900's. A lot of the small town papers had columns describing motoring offences by local citizens. One of them (in 1910) was for a guy prosecuted for "1 armed driving" for which he was fined $2. Whether they meant he had only one arm, or was driving with one arm on the window ledge a la "Roman Holiday", they didn't say.