My commute is about 5.5 km and all urban, and I've found myself using it for some sort of HIIT. On my way to college it's all slightly downhill, and on the way back, therefore slightly uphill. The two km closest to home I go more or less like on my training rides, on the upper limit of my confort zone: since there is relatively little traffic and almost no traffic lights I can keep a semi-constant speed. Then, when I get to the modern part of Barcelona (my neighbourhood used to be a town on its own until the early twentieth century), around 2 km of 100-odd metre blocks, so either I have to ride fast to keep on the green wave or I have fun doing short bursts and rest at the traffic lights. Then I have around 1.5 km on the old part of Barcelona, with a bit of cobbles, less traffic lights and I get into a park to ride a bit on dirt on the last part.