Traffic Jam

Today I had to go to downtown and it has had been a while since the last time that I had to do that... I last went there by car.  --> (it's more natural to say "since I last went there", rather than saying "the last time I had to do that" - you are also right, but it sounds funny to a native)

And I had forgotten how terrible the traffic is. No matter what time it was someone took his car to move around the city, one is away going to be   you would get stuck in the traffic jam. ---> (personally, I like to keep my sentences simple, so that's why I changed your sentence to "no matter what time it was, you would get stuck in traffic")

I'm kind of surprised of how many cars exists in my city. Actually what had astonished me the most was the fact that the majority of cars I've seen had only one single person inside it. Think about that for a minute,. We had tons of tons of car every single day flooding the streets and the high amount most of those cars only carry a single person. It means that on the average, one person occupies approximately 7 square meters (taking into account space a common car takes).

In my opinion that's too much. Don't get me wrong, I think cars are useful andare should be used - in some specific aspects. But we can see clearly that the way cars are currently being used this model of living is unmaintainable unsustainable.