The Bulk of your transportation needs are fulfilled with roads of one flaover or another.
More expensive then streets (some of which are built for you when you zone an area), Roads have the benefit of higher capacity and speed limit. If you notice crowding and gridlock on your streets, you can upgrade them by building Road right on top.
These are cheap, comparatively, but Streets have the lowest capacity and speed limits of all your road options. When Zoning, Streets are automatically built by zones so that Sims can drive out of their parking lots and driveways.
These take up twice as much space as streets or roads, and are more expensive. However, they have the advantage of higher capacity and can be used to replace roads and can be downgraded from highways.
A road that only allows you to travel in one direction. When you build them, the way you drag is the way the traffic will go. Make sure to have another one way road heading in the opposite direction or you'll have a problem.
The ground highway is a large highway system offering more capacity and less gridlock than the avenue. However, it rests on the ground becoming more obtrusive (but cheaper) than an elevated highway. Overpasses built on ground highways will have the road passing over the highway in an automatically created bridge.
The Elevated Highway is more expensive but less obtrusive than the Ground Highway. All overpass sections will have roads and rails passing under the highway (with the exception of elevated rails). The capacity and gridlock remain the same as ground highways. Ground highways are easily able to automatically convert to elevated highways with the joining of both strains at the ends.
The cloverleaf is a form of highway intersection consisting of simple on ramps for one highway turning right into the other highway, and the looping 270 degree on ramp for one highway turning left onto the other highway. Therefore it assumes the shape of a 4 leaf clover. This can be placed from the menu item with both elevated and ground highways connecting to the ends, OR it can be automatically created when one builds a strand of highway crossing over a strand already in place.
Overpass on ramps allow the connection of crossing roads onto and off of the highways they cross. This can be selected manually from the menu and apply to all forms of highways. They can also be created by crossing the road and the highway. A prompt asking the player to build on ramps appears once a road is crossed over or under a highway, to which the player can accept or decline building.
Similar to the overpass on ramp, however this is built between a highway and a road parallelling one another. No automatic prompt appears when one builds a road parallel to a highway, and therefore, the side on ramp must be selected from the menu.
A directional T interchange uses flyover/under ramps in all directions at a three-way interchange. A semi-directional T does the same, but some of the splits and merges are switched to avoid ramps to and from the passing lane. Directional T interchanges are very efficient, but are expensive to build compared to other three-way interchanges. They also require three levels, which can be an eyesore for local residents. However, the T-interchange is perferred to a trumpet interchange because a trumpet requires a loop ramp which speeds can be reduced to as much as 25 MPH (US), but flyover ramps can handle much faster speeds.