What is capacity and level of service?

The term capacity is associated with transportation engineering. Capacity helps to understand how many pedestrians or vehicles can be handled by a transport facility. Level of service is also a term associated with transportation engineering. The level of service indicates the performance of the transport facility.

Capacity

Capacity is an important parameter in transportation engineering. As mentioned earlier, capacity helps to understand the pedestrians and vehicle handling capacity of a transport facility. The capacity and all its factors are well elaborated in the Highway Capacity Manual (HCM). As per HCM, capacity is defined as the maximum howdy rate at which the vehicles or people are expected to traverse a point of a lane or a roadway during a given time under a prevailed roadway and traffic condition. The capacity is described as an hourly rate of flow as per the guidelines of HCM, but in practice, the 15 minutes flow is considered while calculating the capacity. The factors affecting capacity are given below:

• Traffic conditions.
• Roadway conditions and the geometric conditions of the road like widths and the alignments of the road.
• Traffic signal timings.

Level of service

The level of service is an important parameter in transportation engineering. As mentioned earlier, the level of service is a procedure described by the Highway Capacity Manual (HCM) to indicate the performance of traffic service. HCM suggested six different levels of services, namely, A, B, C, D, E, and F. The 'A' denotes the best level of service and 'F' denotes the worst level of service. The factors considered while evaluating the level of service are operating speed, travel time, traffic interruptions, driving comfort, economy, safety, traffic volume, and so on. The 'A' level of service is considered to exist when the vehicles can run at free speed or the speed of their own will and overtake the slower vehicles at their desire. The level of service decreases to the values B, C, D, and E with the increase in traffic volume and volume to capacity ratio of vehicular traffic. With the decreasing level of service, the flow rate of vehicles reduces, resulting in congestion of vehicles. At the lowest level of service 'F', the flow rate decreases to values much lesser than the level of service of the road, resulting in no flow condition, and hence, traffic jam. Hence, proper measures should be taken while designing a road that the road should offer the best level of service.

Intersections

Intersections are the areas shared by two more roads. Intersections are the critical points in any single lane or multilane road system as vehicles, pedestrians, bicyclists moving in different directions occupy the same space for lane changes, direction changes, and traversing each other. As per the Transportation Research Board, proper measures should be taken by traffic engineers while designing the intersections.

Capacity and Level of control at an intersection

The control at intersections is done at three levels, depending upon the traffic conditions. The three levels are passive control, semi-control, and active control.

Passive control

The passive control procedure is applicable when the traffic volume is less and no control is required over the vehicles and pedestrians. In passive control, the drivers are expected to follow the basic rules of driving, such as driving on the left side of the road and following the signs and markings. The passive control contains traffic signs, guide signs, warning signs, and markings to get some amount of control. At the intersection of major and minor roads, three types of stop controls are used, namely, Give-way stops control, Two-way stop control, and All-way stops control.

• In Give-way stop control, the driver on the minor road is expected to slow down and let the driver on the major road cross the intersection.
• In two-way stop control, drivers on two minor roads should see each other and cross the intersection without conflict.
• In all-way stops control, stop signs are put on the intersections. The drivers at the intersection are expected to stop the vehicles and the vehicle at the right approach is given priority to cross first over the vehicle at the left approach.

Semi control

The semi-control procedure is applicable for moderate traffic zones. In this control, the drivers are guided on a small scale to avoid conflict. There are two methods used for semi control, channelization and traffic rotaries.

Channelization

Channelization is the process of constructing islands into intersectional areas. This reduces the total conflict area. The channelization helps in directing the traffic to different channels and physically separating the vehicles and pedestrians. A proper geometric design is required to execute accurate channel intersections. The different types of channelized intersections include partial TEE channelization, complete TEE channelization, complete Cross channelization, partial Skew channelization, and so on.

Traffic rotaries

Traffic rotaries are the infrastructures that are enlarged road intersections where all the vehicles are forced to move around a large central island in a clockwise direction before the vehicles can weave out in their desired direction. The main motive behind providing a traffic rotary is to reduce the area of conflict and provide clearance to prevent unnecessary stopping of vehicles and pedestrians. The limitation of traffic rotaries is that they are expensive to construct.

Active control

The active control procedure is applicable for heavy traffic vehicle saturation zones. In this control, the vehicles and pedestrians are forced to follow the path as suggested by the traffic regulating agencies. There are two methods used for active control- traffic signals and grade-separated intersections.

Traffic signals

Traffic signals are used to regulate the traffic in high vehicular traffic volume areas. The signals allow the headway movement of traffic only from one road or channel and restrict the flow from all other roads. The phases of signals can be two or more, depending upon the traffic conditions. The signals can operate in two modes, fixed time signals, and vehicle actuated signals. In fixed time signals, the cycle time, number of phases, and the intervals of each signal is fixed. Each next cycle of the signal is the same as that of the previous cycle. The vehicle actuated signals perform based on traffic volume. Vehicle detectors are placed at each intersection. The detectors sense the vehicular traffic and then adjust the timer as per the traffic volume.

Grade separated intersections

Grade-separated intersections cause the least delay to the crossing traffic. The grade separation is achieved through a vertical level. Separation of intersecting roads is done using a bridge and thus maximum conflicts are avoided. the bridge can be provided as a flyover or underpass depending upon the conditions.

Signalized intersections and unsignalized intersections

Signalized intersections and un-signalized intersections are important points in the traffic control system. Various measures are taken in the capacity analysis of a signalized intersection. The most common is the average delay per vehicle, number of vehicles, and queue length (queuing). Delay is the measurement of the extra time taken by vehicles and pedestrians to clear the intersection. Queue length is important in measuring the number of vehicles passing through a section in a given time. Among the three measures, the delay measurement is very important. Delays are categorized into five types: Stopped time delay, Approach delay, Travel time delay, Time in queue delay, and Control delay.

1. Stopped time delay is the delay caused as the vehicle is stopped till another vehicle passes.
2. Approach delay is the stopped delay along with the time lost due to the deceleration of vehicles.
3. Travel time delay is the difference between the expected time of a driver to cross an intersection and the actual time taken to cross the intersection.
4. Time in queue delay is the time required by the vehicle to wait in the queue and then cross the intersection.
5. Control delay is the delay caused to the vehicle due to control devices such as signals and stops signs.

Context and Applications

The capacity and level of service at the intersection is useful for students undergoing the following courses:

• Bachelors in Civil Engineering
• Masters in Civil Engineering
• Masters in Technology (Transportation)

Practice Problems

1. Which of the following terms indicates the performance of the transport facility?

1. Capacity
2. Channelization
3. Level of service
4. Intersection

Answer: Option c

Explanation: Level of service indicates the performance of the transport facility.

2. HCM is the acronym for?

1. Highway Capacity Manual
2. Highway Construction Manual
3. Highway Content Manual
4. Highway Creative Manual

Answer: Option a

Explanation: HCM is the acronym for Highway Capacity Manual.

3. Which of the following is not a factor affecting capacity?

1. Traffic conditions
2. Roadway conditions
3. Traffic signal timings
4. Pavement type

Answer: Option d

Explanation: Pavement type is not a factor affecting capacity.

4. Which of the following is the process of constructing islands into intersectional areas?

1. Channelization
2. Passive control
3. Active control
4. Grade separation

Answer: Option a

Explanation: Channelization is the process of constructing islands into intersectional areas.

5. How many different levels of services do HCM suggest?

1. Four
2. Six
3. Seven
4. Five

Answer: Option b

Explanation: HCM suggests six different levels of services.