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Chapter 21
Capacity and Level of Service LOS
21.1 Introduction
Often it is required to ascertain how much a transport facility can accommodate. Such in- formation is useful in the design of traffic facility. Capacity analysis helps in answering the question. It is a quantitative assessment of the ability of a traffic facility to handle vehicles or people for which it is designed. A related question is, what is the performance level of the system at various operating conditions. Or in other words, how good is the operation of the traffic facility. Level of Service analysis tries to answer this question which is essentially a qualitative analysis. Capacities and Level of Services are therefore closely related analysis of a traffic facility.
21.2 Capacity Concepts
Capacity of a transport facility is defined as the maximum number of vehicles, passengers, or the like, per unit time which can be accommodated under given conditions with a reasonable expectation of occurrence. The Highway Capacity Manual(2010) defines the capacity as the maximum howdy rate at which persons or vehicles can be reasonably expected to traverse a point or a uniform segment of a lane or roadway during a given time period, under prevailing roadway, traffic and control conditions. Several observations can be made from the above defi- nition. Although capacity is the maximum howdy rate, in many situations the break 15 minute flow rate is expressed as the capacity. The above definition also contains the term “reasonably expected” to account for the variation in traffic and driving habit at various location. However, it can be termed as a probabilistic measure. Further, analytical derivations are possible for getting the maximum flow rate, seldom it is achieved in the field. However, capacity measures are often empirically derived. Capacity is usually defined for a point or a uniform segment where operating conditions do not vary.
The capacity measure depends on these operating conditions. The first is the traffic condi- tions and the factors that influence the capacity includes vehicle composition, turning, move- ments, etc. The second factor is the roadway conditions and it includes geometrical character- istics such as lane width, shoulder width, horizontal alignment, vertical alignment. The third factor is the control conditions such as the traffic signal timings, round-about characteristics. It is also to be noted that the above capacity definition holds good for a point or at a section of the road having uniform control conditions. Another aspect of the above capacity definition is the expression that the maximum flow rate which accounts for the worst 15 minutes traffic within the peak hour traffic. Lastly the term reasonable expectancy indicates that the capac- ity measure is probabilistic and not an analytically derived deterministic value. The capacity measure is probabilistic, for it accounts for the unexplainable variation in traffic and diverse driving characteristics.
21.3 Level-of-Service(LOS) Concepts
Level-of-Service(LOS) of a traffic facility is a concept introduced to relate the quality of traffic service to a given flow rate. Level-of-Service is introduced by HCM to denote the level of quality one can derive from a local under different operation characteristics and traffic volume. HCM proposes LOS as a letter that designate a range of operating conditions on a particular type of facility. Six LOS letters are defined by HCM, namely A, B, C, D, E, and F, where A denote the best quality of service and F denote the worst. These definitions are based on Measures of Effectiveness(MoE) of that facility. Typical measure of effectiveness include speed, travel-time, density, delay etc. There will be an associated service volume for each of the LOS levels. A service volume or service flow rate is the maximum number of vehicles, passengers, or the like, which can be accommodated by a given facility or system under given conditions at a given LOS.
21.4 Type of Facilities
HCM has developed the capacities standard and LOS measure for various facilities. Each traffic facility has its own unit for the capacity and measure of effectiveness for each item will also vary. The traffic facilities can be divided into three, namely: the uninterrupted facilities, interrupted facilities, and others. Interrupted facilities include freeway (basic freeway, weaving sections, and ramps), multi-lane highways (unidirectional), two-lane highways(bidirectional). Freeways normally have density as the measure of effectiveness, while multi-lane and two-lane highways
V/C Ratio^ 1.
E F
D
C
B
A
Operating Speed
Figure 21:1: The LOS of a Mid Block Section
Table 21:1: The LOS of a Mid Block Section LOS Quality Speed V/C Description (kmph) A Free-flow 80 0.6 High level of physical and psychological comfort B Reasonable 70 0.7 Reasonable level of free-flow physical and psychological comfort C Near 60 0.8 Local deterioration free-flow possible with blockages D Medium 50 0.85 Non-recoverable flow local disruptions E At capacity 40 0.9 Minor disturbances flow resulting breakdown F Congested 15 1.0 Break down of flow flow capacity drops
21.6 Conclusion
In this lecture the concepts of capacity and LOS is presented. Capacity is a quantitative measure, whereas LOS is a qualitative measure. Capacity defined for various traffic facilities considering the traffic, geometric and control condition and obtained from field observation. LOS on the other side is assigning quality levels of traffic based on performance measure like speed, density, etc. Together, the concepts gave planner a valuable tool in designing and evaluating various traffic facilities.
21.7 Questions
21.8 Acknowledgments
I wish to thank my staff Ms. Reeba in typesetting the materials for this lecture. I also wish to thank several of my students and staff of NPTEL for their contribution in this lecture.
