Question

People grow trees nearby highways why?

Answer 1

People grow more trees nearby highway because near the highway more pollution is there.

Answer 2

The Green Highways (Plantation and Maintenance) Policy, for planting trees on national highways has been finalised. Under this policy, one percent of the cost of national highway development projects will be set aside to plant trees on 97,000 km of existing national highways and around 40,000 km of new roads that we will be added to the network over the next few years. 
I Hope it helps you