2006 Prius, good MPG on the motorway, poor locally

Hi

I have a 2006 Prius, had it for a few years, for the past 4-5 months i've been getting poor MPG, around 30MPG locally, used to get around 45-50 locally.
I took it for a long drive on the motorway and got 50+MPG.... but locally I'm getting no where near what i should,
The battery is charging, and isnt loosing charge any quicker than it used to 3 years ago.
I've changed Air filter, Pollen/Cabin filter, oil and oil filter. Changed the plugs around 2 years ago, and have done around 10k since changing, It has low mileage around 105k

Any ideas why locally I dont get the correct MPG but on the motorway i did?