Motorists in Portsmouth are having to limit their speed to 20mph in nearly all residential streets from Tuesday.
Portsmouth council said residents supported the scheme
The new speed limit has been introduced over the past two years to protect all users of the city's narrow, congested streets, the council said.
The latest and final areas to join the scheme are the city centre and historic Old Portsmouth.
About 1,200 city streets are covered by the lower limit although most of the main routes are not affected.
Portsmouth City Council claims its adoption of the 20mph limit is "a first" in terms of the extent of the area covered but other towns and cities in the UK have introduced similar schemes.
Councillor Alex Bentley, the council's executive member for traffic and transportation, said: "The vast majority of Portsmouth people have supported us every step of the way, and have been keen to get their streets covered.
"They know it's dangerous to drive faster than 20mph on our narrow residential roads, where cars are parked on both sides.
"We're sending a message to the small number of reckless drivers that they must drive safely.
"If our streets are safer, the more people are likely to walk or ride a bike, which is good for health, congestion and the environment."