Princess Road and Princess Parkway in Manchester will be lowered from 40mph to 30mph for an 18-month trial after a proposal was approved by councillors.
There have been two deaths and 84 crashes over the past three years.
A friend of Ian O'Mara, who died when he was hit by a coach on a pelican crossing in 2015, helped set up a petition to change the speed limit.
A coach driver who hit the dog walker in Princess Road was given a suspended sentence after admitting causing death by careless driving.
The change to the speed limit was passed by Manchester City Council on Wednesday, although a date for when it comes into force is yet to be confirmed.
Princess Parkway merges into Princess Road and stretches 3.7 miles (6km).
Campaigner Sam MacWilliam, from Chorlton, said: "The road is very important for people coming into the city, but they need to understand that they are coming off the motorway and into an area where people live.
"There are schools and churches here."

About 5,500 homes and businesses remain without power on Wednesday evening.
The places worst affected by the storm are Wicklow, New Ross, Athlone, and Inistioge in County Kilkenny, according to Irish state broadcaster, RT??.
In Midleton, County Cork, the Irish Army helped to evacuate about 12 family homes after a river overflowed. Several people were also rescued from cars.
They included a woman and two children who were rescued from a car in Glanmire, County Cork.
The main road from Cork to Kerry, the N20, remains badly flooded in places.
The storm also led to rail service disruption in some areas, especially around Cork, where passengers were bussed between Tralee and Killarney and Mallow due to subsidence along the rail track.
Some sailings by Stena Line and Irish Ferries have been cancelled but sailings of P&O Ferries are running to schedule.
Environment Minister Alan Kelly and Office of Public Works Minister Simon Harris visited Athlone on Wednesday afternoon, where Storm Frank has caused extensive flooding, adding to the problems caused by Storms Desmond and Eva.
Mr Harris said Irish Prime Minister (Taoiseach) Enda Kenny would be visiting more places affected by the flooding in the coming days and the flooding issue would be first on the agenda at Tuesday's cabinet meeting.
The speed limit on a major route into a city has been reduced following a campaign launched after a man's death.
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Storm Frank has caused flooding and disruption to travel and electricity supplies in the Republic of Ireland.