A law-abiding, 91-year-old woman has tied herself to a tree to stop it being chopped down.

Grandmother of three, Isabel Mackenzie says hundreds of trees along Victoria's Western Highway, some more than 600-years-old, will be cut down because VicRoads wants to widen the road in Buangor, east of Ararat.

"All my life I've been a strictly law-abiding person, but because of this unconscionable damage, I feel impelled to act," Mrs Mackenzie said.

The protest is small - there is one other protester tied to the tree with Mrs Mackenzie and two others supporting them on the roadside.

However, they are making their point with passing motorists beeping, waving and even stopping to voice support, Mrs Mackenzie said.

A bus load of tourists on their way to Adelaide even stopped and helped out by holding up placards.

The police have been "perfectly civilised" and "exchanged pleasantries" with them when they visited the protest site.

VicRoads said it was trying to minimise the environmental impact of the road duplication.

"We're looking at each tree on a case-by-case basis and where we can we are avoiding the impact, building around certain trees or relocating them, however, in some cases, it's simply unavoidable," said Michael McCarthy, director of the Western Highway project.

VicRoads says it will plant 12,000 new trees and 50,000 other plants when the construction is complete.

The protest ended peacefully just before midday when their supporters unlocked them from the tree.