Safety and Efficiency of a Roundabout

One of the key considerations in designing a roundabout is to ensure both safety and efficiency. Safety is crucial to prevent accidents and protect the lives of road users, while efficiency aims to facilitate smooth traffic flow and reduce congestion. To achieve these goals, various factors need to be taken into account, including the entry and exit radii of the roundabout.

Entry Radius
The entry radius refers to the curvature of the road leading into the roundabout. Increasing the entry radius can enhance the safety of the roundabout in the following ways:

1. Reduced Vehicle Speed: A larger entry radius allows vehicles to enter the roundabout more smoothly, without the need for sharp turns. This results in reduced vehicle speed, which in turn enhances safety by giving drivers more time to assess the traffic situation and make appropriate decisions.

2. Improved Visibility: With a larger entry radius, drivers have a better line of sight as they approach the roundabout. This increased visibility allows them to observe any approaching vehicles or pedestrians and adjust their speed accordingly, reducing the risk of collisions.

3. Reduced Lane Changing: A wider entry radius allows for the provision of additional entry lanes. This reduces the need for vehicles to change lanes abruptly upon entering the roundabout, minimizing potential conflicts and enhancing safety.

Exit Radius
The exit radius refers to the curvature of the road as vehicles leave the roundabout. Increasing the exit radius can contribute to the efficiency of the roundabout in the following ways:

1. Smooth Traffic Flow: A larger exit radius allows vehicles to exit the roundabout more smoothly, without the need for sharp turns. This promotes a continuous flow of traffic, reducing the likelihood of congestion and delays.

2. Reduced Queuing: A wider exit radius provides more space for vehicles to maneuver and merge into the appropriate lanes upon exiting the roundabout. This reduces the potential for queues to form, as vehicles can exit more efficiently without disrupting the flow of traffic.

3. Improved Capacity: By increasing the exit radius, the roundabout can accommodate a higher volume of traffic. This increased capacity enhances the overall efficiency of the roundabout by reducing congestion and improving traffic flow.

Conclusion
In summary, the safety within a roundabout can be increased by increasing the entry radius, which leads to reduced vehicle speed, improved visibility, and reduced lane changing. On the other hand, the efficiency of a roundabout can be enhanced by increasing the exit radius, which promotes smooth traffic flow, reduces queuing, and improves capacity. Therefore, the correct answer is option 'C' - decreasing the entry radius and increasing the exit radius.