From what I understand, roads are only used between garages or buildings that have a parking entrance. Is there a benefit to connecting other buildings with roads? Can citizens get "dropped off" at their job even if it doesn't have its own parking entrnace? If so, do they have to then walk to the nearest garage to get home (or elsewhere)?
I have seen citizens get "dropped off" at work (most notably construction workers), so you would want roads that leads from where your citizens lives to the areas where they are most likely to visit to cut down on time.
However, when its time to go home, they will not get picked up and has to walk to the nearest garage for transportation.
As you build your road network, try to keep congestion low as I've also seen huge traffic backups in major areas which I'm sure lowers productivity. You wouldn't want your minions to spend more time in traffic and less time in the sweat shops, after all.
Also, each garage seems to be able to support 150ish cars, if there are higher demands than that you will receive a message that says the garage is overloaded. Try to plan ahead so there's not just one garage servicing your "downtown" area, for example.