Drivers are to get a 10-minute grace period when their time runs out at private car parks.
It means those who overstay their paid parking period by a few minutes should now avoid a fine.
The changes are coming in after industry bodies the British Parking Association (BPA) and the International Parking Community (IPC) published a new code of conduct.
However, the AA said it still leaves room for drivers to be ripped off because it misses out "desperately needed" measures such as a cap on charges.
An "appeals charter" and more consistent signage are also in the new code, which the BPA called a "milestone" for "fairer and more consistent parking standards".
The two industry bodies are asking all private parking companies to implement it by 1 October.
IPC boss Will Hurley said it would help protect "the most vulnerable in society, whilst creating consistency and clarity for motorists and continuing to elevate standards across the sector".
Some 9.7 million parking tickets were issued by private firms between April and December last year - equivalent to more than 35,000 per day, according to the RAC Foundation.
A legally-backed code of conduct was due to come in force by the end of last year but was withdrawn by the government after parking firms challenged it.
It would have halved the cap for most parking offences to £50, established a fairer appeals system and banned aggressive language on tickets.
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The AA said the industry bringing in its own code was "somewhat ironic" and the guidelines still left room for unreasonable charges.
"This watered down code of practice falls far short of the standards the AA, government and consumer groups have called for across many years," said head of roads policy Jack Cousens.
"This self-authored code doesn't acknowledge the need to cap charges and remove debt recovery fees.
"These elements are desperately needed from a government-backed code to protect innocent drivers from the sharks running private car parks."
The RAC said the only way to truly stop unfair charges is a code fully backed by the law.
"Drivers shouldn't be fooled into thinking this so-called code developed by the private parking industry itself is the same as the long-delayed official private parking code of practice that is backed by legislation," added the group's head of policy Simon Williams.
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