He could have done that a lot more briefly. Just make sure you have lots of highways and exits to distribute the traffic. His design doesn't even quite include it, because the map forced him to stretch the city over a larger area, but generally, you will end up with one big blob of a city.
Let's say X are the city, gaps are the highways and the colourized are the exits. Instead of building something like this:
XXXXXXXXXXXXXXX
XXXXXXXXXXXXXXX
XXXXXXXXXXXXXXX
XXXXXXXXXXXXXXX
XXXXXXXXXXXXXXX
XXXXXXXXXXXXXXX
You should build something like this:
XXXXX
XXXXXXXXXXX
XXXXXX
XXXXXXXXXX
XX
XXXX
XXXXXX
XXXX
XX
XXXX
XXXXX
XXXXX
XXXXXX
XXXXXXXXXX
XXXXX
XXXXXXXXXXX
This way the traffic comes from different directions (they leave the highway closest to their destination) and has different destinations rather than using the exact same shortest path (that's how the AI works) to get to the other side of the city.