Average speed ?

Ok I have a DAHON speed uno. It’s. Single speed with 48 - 19 gearing. Yes it’s sort of slow like 15 mph is a speed I can keep but with traffic lights and all my average speed seams to be around 10 mph i thought to change the front chainring to something much bigger to up my top speed but idk I love the bike but what is your average speed over 5 or 10 miles thx