A Christchurch mother's concerns have proven justified after a Police sting caught out a large number of traffic offenders near her child's school.

Police monitored the Cranford and Westminster street intersection on Friday, after Rachel Donaldson put up five signs near St Albans School, urging drivers to follow the road rules.

During the one hour sting Police issued 20 tickets, 17 of which were traffic light offences.

Ms Donaldson said she wasn't shocked, but believes that number of offences is outrageous.

"That is what we see every single day at those lights."

"While those numbers are really really high, I'm not really surprised," she added.

She is concerned that there will be an accident at the intersection, and said it could be a "car versus child incident."

"It will be because of running a red light."