ROADS

Road - This is your primary method of transport.  Roads connect zones, allowing them to grow.

Highway - Hiways are faster and more efficient than roads.  Commuters will move from road to onramps, then to hiway; back to onramps, then road again.  Without the roads and on-ramps, hiways are useless.

Tunnel - Tunnels dig through a mountain rather than going over it.  One advantage to tunnels is that zones can be built over tunneled areas, improving land usage.

Onramp - These are necessary for hiways to function.  They can only be placed at a hiway/road juncture.

Bus Depot - Depots provide rapid, low traffic transport.  Up to half of your city's commuters will use these depots if they are well placed.