For highway lowering the speed may be effective but lowering the speed limit from 50 to 30 won't stop drivers from going 50 unless the road is designed for lower speeds. So long as lanes are wide and there are little obstructions for drivers to worry about hitting, such as bumpouts, boulevards, etc., they will go as fast as they feel comfortable unless there is a cop behind them.

Edit: 50kmh to 30kmh I don't know what that is in freedom miles