How to internalize environmental issues in North- South highway to be constructed in Sri Lanka?

First step should always be to quantify the environmental impacts. This could be done e.g. by carrying out a Life Cycle Assessment (LCA). LCA shows where impacts occur, and by what they are caused. Then these impacts can be analysed, and measures to avoid/reduce the environmental impact can be taken. But the first step should be the identification of the actual impact.