I dont think carrying a knife at night is a good idea. You should only ever pull out a weapon if your going to use it. Quite often the person who brings the weapon to the fight is the one who ends up getting hurt by it.

That said, i cycle through the town centre and 2 large parks to get home each night, and i carry a large wrench for running repairs and self defence. I have only had to use it once but it gives me a bit of peace of mind.