A police officer who went undercover as a thief was able to steal four bicycles in broad daylight without any passers-by calling 999.
PCSO Neil Spiring even wore a fancy dress costume to carry out one of the thefts in a busy street, but still his 'crime' went unnoticed.
Avon and Somerset Police, which carried out the test as part of its Operation Relentless drive, said more than 50 people saw the thefts but none dialled 999.
PCSO Spiring used bolt croppers and brute force to steal the bikes as members of the public walked past, and police have released CCTV video footage of the tests to show the astonishing results.
Inspector Keith Rundle said: "We're not suggesting that anyone become a 'have-a-go hero' but we're asking people to call 999 and report bike theft as soon as they see it happening so we can catch the bike thieves in the act.
"We are constantly on the lookout for bike thieves but we also need the community to be vigilant and to call us if they see a crime happening.
"If you see anything suspicious, call 999 and let us know. The faster you let us know the more likely we are to be able to catch the thief."
Watch the video footage from Avon and Somerset Police below...
TOP POLICE TIPS TO KEEP YOUR BIKE SAFE
1) Invest in a good lock - it might seem expensive but it could save you hundreds in the long run in replacing stolen parts or even an entire bike.
2) Always use two different lock types.
3) Ensure that all parts of the bike that can be removed are taken with you or locked to the frame.
4) Where possible, lock to an official stand that is firmly in the ground.
5) Be security savvy - Bike thieves know their locks (and how to break them) so should you.
6) Consider electronic tagging.
7) Register your bike and all your property at www.immobilise.com.