Motorway speeds decreased?

The Irish government is looking to decrease the speed at which cars travel on the motorway. At the time the speed limit is 100 km/h. If the government decrease this speed, which they said they would do because "It uses less fuel". But would it actually use less fuel?

What I think is going to happen here is people are going to spend more time on the motorway, so it would use MORE fuel.

I don't think the government has really thought this through, and I don't know if other governments are planning on doing the same thing. Let's just hope that I'm right.