The Automobile Association (AA) questioned Aberdeen City Council's policy after it issued 16,801 penalty charge notices within six months of a new bus lane enforcement being in place.
The move was introduced in the city in March, but the AA said the amount of drivers caught by September was on an "industrial scale".
A spokesman said: "We understand they need to keep the bus lanes clear to improve the flow of traffic, but the amount of money suggests that people are being caught on an industrial scale. If this level of fines continues there needs to be a very serious review, because this is a massive amount of money.
"The decision rests in the hands of someone who thinks about the amount of money, rather than assessing whether it's a sensible decision."
The spokesman said the policy was vital to ensure road safety and the flow of traffic, but the system often caught out drivers unfamiliar with local regulations.
The initial fine is £60, cut to £30 if paid in 14 days.
An estimated £350,000 has been collected since March, but the council was unable to provide a figure for outstanding penalty charge notices.
Finance convener Willie Young insisted the policy was not a tax on motorists, but designed to speed up bus and taxi travel times.
He said: "The first thing to say is that money from fines is ploughed straight back into infrastructure and transport improvements.
"Secondly, we have to keep the buses and taxis moving."