Level of service

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Level of service is a measure by which transportation planners reckon the quality of service on transportation devices, or transportation infrastructure, generally linked to transportation time (the shorter, the better) and thus to speed. ^[1] Level of service may be used for other public facilities as a tool to measure changes in condition or availability, such as water supply.

The transportation LOS system uses the letters A through F, with A being best and F being worst. LOS A is the best, described as conditions where traffic flows at or above the posted speed limit and all motorists have complete mobility between lanes. LOS A occurs late at night in urban areas, frequently in rural areas, and generally in car advertisements.

B is slightly more congested, with some impingement of maneuverability; two motorists might be forced to drive side by side, limiting lane changes. LOS B does not reduce speed from LOS A.

LOS C has more congestion than B, where ability to pass or change lanes is not always assured. LOS C is the target for urban highways in many places. At LOS C most experienced drivers are comfortable, roads remain safely below but efficiently close to capacity, and posted speed is maintained.

LOS D is perhaps the level of service of a busy shopping corridor in the middle of a weekday, or a functional urban highway during commuting hours: speeds are somewhat reduced, motorists are hemmed in by other cars and trucks. In busier urban areas this level of service is sometimes the goal for peak hours, as attaining LOS C would require a prohibitive cost in bypass roads and lane additions.

LOS E is a marginal service state. Flow becomes irregular and speed varies rapidly, but rarely reaches the posted limit. On highways this is consistent with a road over its designed capacity.

LOS F is the lowest measurement of efficiency for a road's performance. Flow is forced; every vehicle moves in lockstep with the vehicle in front of it, with frequent drops in speed to nearly zero mph. Technically, a road in a constant traffic jam would be below LOS F. This is because LOS does not describe an instant state, but rather an average or typical service. For example, a highway might operate at LOS D for the AM peak hour, but have traffic consistent with LOS C some days, LOS E or F others, and come to a halt once every few weeks. However, LOS F describes a road for which the travel time cannot be predicted.

The Highway Capacity Manual and AASHTO-Geometric Design of Highways and Streets ("Green Book") list the following levels of service:
A= Free flow
B=Reasonably free flow
C=Stable flow
D=Approaching unstable flow
E=Unstable flow
F=Forced or breakdown flow

Additionally, the Highway Capacity Manual (2000) defines Level of service at intersections as a function of the average overall wait time for a vehicle to pass through the intersection. This way, LOS can be quantitatively measured at any intersection.

LOS	Signalized Intersection	Unsignalized Intersection
A	≤10 sec	≤10 sec
B	10-20 sec	10-15 sec
C	20-35 sec	15-25 sec
D	35-55 sec	25-35 sec
E	55-80 sec	35-50 sec
F	≥80 sec	≥50 sec

The level of service characterizes the operating conditions on the facility in terms of traffic performance measures related to speed and travel time, freedom to maneuver, traffic interruptions, and comfort and convenience."

The above grading refers to highways; however, some professors in urban planning schools have proposed measurements of levels of service that take public transportation into account. Such systems would include wait time, frequency of service, time it takes to pay fares, quality of the ride itself, accessibility of depots, and, perhaps, other criteria as well.

LOS can also be applied to surface streets, to describe major signalized intersections. A crowded four-way intersection where the major traffic movements were conflicting turns might have an LOS of D or E. At intersections, queuing time can be used as a rubric to measure LOS; computer models given the full movement data can spit out a good estimate of LOS.

In the past, some planners have aimed for an "A" Level of Service (and many still do in rural areas), but many transportation planners (especially proponents of public transit) recommend aiming for a "C" level of service (particularly in urban areas), one that would slow cars down and make roads safer for pedestrians (thus increasing the desirability of public transit if such transit has its own rights-of-way). To that end, transit-favoring planners recommend increasing population density in towns, narrowing streets, restricting car use in some areas, providing sidewalks, and making the scenery interesting for pedestrians.

A level of service standard has been developed by John J. Fruin, PhD., for pedestrian facilities. The standard uses American units and applies to pedestrian queues, walkways, and stairwells.

References:

1. ^ Mannering, Fred L.; Walter P. Kilareski, Scott S. Washburn (2004-07-26). Principles of Highway Engineering and Traffic Analysis. John Wiley & Sons; 3 edition, pp. 170-219. ISBN 0-471-47256-5. 

http://www.walksf.org/pedestrianLOS.html
http://www.dot.state.fl.us/Planning/systems/sm/los/pdfs/pedlos.pdf
http://elevatorbooks.stores.yahoo.net/pedplanandde.html