The speed limit on a section of Greenford Road will be reduced to 20mph as part of an upgrading scheme.
Work on installing new cycling facilities, trees and benches between Sudbury Hill Harrow station and Ealing shopping parade started earlier this month.
It means motorists will be limited to 20mph until the scheme's completion, expected in December this year.
Ealing Council has not yet said when the limit will come into force.
The upgrades are part of Transport for London's (TfL's) Healthier Streets initiative, in partnership with Ealing Council and Harrow Council.
Councillor Bassam Mahfouz, Ealing’s cabinet member for environment transport and leisure, said: "The council is working in partnership with Harrow and TfL to make these significant upgrades.
"There is a range of improvements planned that will make Greenford Road and Sudbury Hill High Street safer and more attractive places for local people.
"The changes will also see a boost for the local economy and the vibrant local high street.
"The council is committed to working proactively with local businesses and residents to minimise any disruption during the period of development."
The scheme will result in the introduction of step-free access to Sudbury Hill Harrow station from both sides of the street.
The stop-and-shop parking will change from 30 minutes to one hour on the Ealing side, to match the current provision on the Harrow side.
There will also be segregated cycling facilities on the Ealing side of the borough boundary.
Enhanced lighting has been promised along with new, improved footways.
Cllr Graham Henson, Harrow’s cabinet member for environment, crime and community safety, said: "We welcome this much needed upgrade which will bring many benefits for both our residents and businesses in the area - this includes improvements to both transport networks and the environment.
"Our joint partnership with Ealing Council and TfL will help to keep both boroughs moving, make the area a more vibrant and attractive place to live and work, while also encouraging people to walk, cycle and use public transport for healthier living."
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