Aberdeen city centre has become a 20mph zone for all traffic.
A 20mph speed limit sign in Aberdeen city centre
It is the first city in Scotland to bring in the blanket speed limit but business leaders are concerned about how trade could be affected.
The new speed restrictions have come into force on about 100 streets in and around Union Street.
The city council said the new limit would significantly improve pedestrian safety and make the centre more attractive as a destination.
Aberdeen City Council deputy leader Ian Yuill said the move was an important step towards improving pedestrian safety in the centre of Aberdeen.
He said: "Many thousands of people use the city centre streets on a daily basis and we believe it's important that traffic travels at the appropriate speed."
One motorist who supported the move said it was difficult to drive over 20mph in parts of the city centre anyway.
Another said too many motorists were driving too quickly and, with so many pedestrians in the city centre, the new speed limit was an important safety measure.
Grampian Police Supt Harry Thorburn said he was in favour of the move.
"I welcome the introduction of the zone and see it as a fundamental step towards reducing road casualties in Scotland," he said.
"All the available evidence points to a lowering of speed thresholds, markedly reducing the severity of injury, and I think it's a very welcome step."
However, the City Centre Association, which represents businesses and property owners in the area, doubted whether the measures were enforceable.
Spokesman John Michie said: "A lot of our customers come from the country area and if the message coming out is that Aberdeen is unattractive to car users then that's not the message we want."
Sue Nicholson, head of campaigns at the RAC Foundation, said city-wide limits were "potentially risky".
"Speed limits within the vicinity of schools and residential areas are well complied with because drivers know that the safety of kids is the justification for that," she said.
"But I think blanket bans are potentially a bit more risky and questionable.
"You've got the possibility that people will simply divert to peripheral roads and divert any possible collisions or casualties to these alternative routes."
Ms Nicholson said more crashes could be caused by drivers "watching their speed dials so carefully" and also raised doubts over the law's implementation.
"How do we enforce these limits?" she said.
"The police are already overstretched so we're looking at yet more cameras on the roads.
"Wouldn't we get a greater reduction in accidents if we targeted dangerous driving like tailgating and drink driving?"