Police found dozens of motorists left their cars unlocked during an operation in Croxley Green.
Officers tried a total of 1,155 car door handles over the space of two hours during an operation on Thursday, February 17.
Hertfordshire Constabulary says 79 were not locked.
The force is now urging people to make sure to secure their vehicles as criminals have been targeting those that are unlocked and stealing them or taking property from inside.
Sergeant Adam Johnson said: “We want to educate and advise residents around securing their vehicles, in doing so highlighting how many unsecure vehicles were in their community and at risk of theft.
“Please always remember to lock your vehicle and check your vehicle has locked. Move any valuables out of sight and ideally take them into your house with you.
“As we continue to ensure the safety of residents, please be assured we are out there with the aim of deterring criminal activity from the area and keeping residents and their vehicles safe.
“Your mobile phone, coins for the car park, sunglasses, packs of medication or other items that can earn quick cash are irresistible to the opportunist thief. And it should go without saying that wallets, handbags, purses and credit cards should never be left in an unattended vehicle.”
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