A modern roundabout is a circular intersection where drivers travel counterclockwise around a center island. There are no traffic signals or stop signs in a modern roundabout. Drivers yield at entry to traffic in the roundabout, then enter the intersection and exit at their desired street.
 
Studies by the Federal Highway Administration have found that roundabouts can increase traffic capacity by 30 percent to 50 percent compared to traditional intersections.
 
Modern roundabouts are designed to accommodate vehicles of all sizes, including emergency vehicles, buses, and truck and trailer combinations. In a modern roundabout, drivers enter the intersection by navigating a gentle curve. Drivers yield at entry to traffic already in the roundabout, then proceed into the intersection and exit at their desired street.
 
A main feature of the modern roundabout is a raised central island, the circular shape is designed to control the direction of traffic and reduce speeds. It also reduces the likelihood of t-bone or head-on collisions.
 
The central island of many roundabouts includes a truck apron, a raised section of concrete that acts as an extra lane for large vehicles. The back wheels of the oversize vehicle can ride up on the truck apron so the truck can easily complete the turn, while the raised portion of concrete discourages use by smaller vehicles.
 
In addition to the central island, roundabouts also feature triangular splitter islands to slow and direct traffic. The islands also provide a refuge for pedestrians. This means pedestrians can choose to cross one direction of traffic at a time and have a safe place to wait before crossing another direction of traffic.

Roundabouts are designed to make intersections safer and more efficient for drivers, pedestrians and cyclists. There are two types of roundabouts: single-lane and multi-lane roundabouts.
There are a few key things to remember about driving roundabouts:

• Yield to drivers in the roundabout
• Stay in your lane; do not change lanes
• Do not stop once you are in the roundabout
• Avoid driving next to oversized vehicles 

As you approach a roundabout, you will see a yellow "Roundabout Ahead" sign with an advisory speed limit for the roundabout. 
Slow down as you approach the roundabout, and watch for pedestrians in the crosswalk. 
 
Motorists must yield to pedestrians when entering and exiting a roundabout.
 
Continue toward the roundabout and look to your left as you near the yield sign and dashed yield line at the entrance to the roundabout. Yield to traffic already in the roundabout.
Once you see a gap in traffic, enter the circle and proceed to your exit. If there is no traffic in the roundabout, you may enter without yielding.
Look for pedestrians and use your turn signal before you exit, and make sure to stay in your lane as you navigate the roundabout.
The Pennsylvania Department of Transportation has put together a useful brochure providing General Information and Driving Tips for Motorists using Single-Lane Roundabouts.

In a multi-lane roundabout, you will see two signs as you approach the intersection: The yellow "Roundabout Ahead" sign and a black-and-white "lane choice" sign. You will need to choose a lane prior to entering the roundabout.
You choose your lane in a multi-lane roundabout the same way you would in a traditional multi-lane intersection. To go straight or right, get in the right lane. To go straight or left, get in the left lane. Drivers can also make U-turns from the left lane. 
The graphics below show what turns can be made in multi-lane roundabouts. The arrows in yellow show the movements that can be made from the right lane, and the arrows in green show the movements that can be made from the left lane. 

Once you have selected your lane, watch for pedestrians in the crosswalk as you approach the roundabout.
Motorists must yield to pedestrians when entering and exiting a roundabout.
 
At the dashed yield line, look to your left and yield to drivers already in the roundabout. Remember, in a multi-lane roundabout, you must yield to both lanes of traffic.
Once a gap in traffic appears, merge into the roundabout and proceed to your exit. Look for pedestrians and use your turn signal before you exit. If there is no traffic in the roundabout, you may enter without yielding.
 
The Pennsylvania Department of Transportation has put together a useful brochure providing General Information and Driving Tips for Motorists using Multi-Lane Roundabouts.

Roundabouts are designed to accommodate vehicles of all sizes, including emergency vehicles, buses, farm equipment and semitrucks with trailers. Oversize vehicles and vehicles with trailers may straddle both lanes while driving through a roundabout. 
Many roundabouts are also designed with a truck apron, a raised section of pavement around the central island that acts as an extra lane for large vehicles. The back wheels of the oversize vehicle can ride up on the truck apron so the truck can easily complete the turn, while the raised portion of concrete discourages use by smaller vehicles.
Because large vehicles may need extra room to complete their turn in a roundabout, drivers should remember never to drive next to large vehicles in a roundabout. 

Modern roundabouts are designed to be safer for pedestrians than traditional intersections. Vehicles are moving at a slower rate of speed in roundabouts. 
Crosswalks are also set further back from traffic, allowing drivers more time to react to pedestrians before merging into or out of the roundabout. Triangular islands between lanes of traffic give pedestrians a safe place to wait if they choose to cross only one direction of traffic at a time. 
Pedestrians should look for approaching traffic, then walk through the crosswalk to the triangular island. Before continuing, they should look for traffic entering or exiting the roundabout. When it is safe, pedestrians can continue through the crosswalk. 

Bicyclists can choose to ride through the roundabout with traffic or walk their bicycles through the pedestrian crosswalks -- much like they would in a traditional intersection. Like vehicles, cyclists must obey the rules of the roundabout as they proceed through the intersection. Riders who choose to walk their bicycles may find that some roundabout designs have a ramp onto the sidewalk, which makes it easier for bicyclists to use. 
 
The Pennsylvania Department of Transportation has put together a useful brochure providing General Information for Bicyclists and Pedestrians using Roundabouts.