Level of service
Level-of-service (LOS) is a measure-of-effectiveness by which traffic engineers determine the quality of service on elements of transportation infrastructure. Whilst the motorist is, in general, interested in speed of his journey, LOS is a more holistic approach, taking into account several other factors.
Level of service may be used for other public facilities as a tool to measure changes in condition or availability, such as water supply.
Level-of-Service in the USA
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 at 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. Facilities operating at LOS F generally have more demand than capacity.
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
D=Approaching unstable flow
F=Forced or breakdown flow
LOS for At-Grade Intersections
Highway Capacity Manualdefines level-of-service for signalized and unsignalized intersections as a function of the average vehicle control delay. LOS may be calculated per-movement or per-approach for any intersection configuration; however, LOS for the intersection as a whole is only defined for signalized and all-way stop configurations.
When analyzing unsignalized intersections that are not all-way stop-controlled, each possible movement is considered individually. Each movement has a rank. "Rank 1" movements have priority over "rank 2" movements, which have priority over "rank 3" movements, which have priority over "rank 4" movements. The rank of each movement is as follows, with the "minor road" being the road that is controlled by the stop signs and the "major road" being the road whose through movement moves freely. As for vehicular movements that conflict with pedestrian movements of the same rank, pedestrians have priority:
# Movements of this rank are the through movements on the major road, parallel pedestrian movements, and right turns from the major road. LOS for movements of this rank is trivial, because LOS is determined by control delay. These are "free" movements, and as such the control delay is always zero.
# Movements of this rank include left turns from the major road.
# Movements of this rank include through movements on the minor road, parallel pedestrian movements, and right turns from the minor road.
# Movements of this rank include left turns from the minor road.Movements are analyzed in order of rank (highest rank first), and any capacity that is left over from one rank devolves onto the next rank below. Because of this pecking order, depending on intersection volumes, there may be no capacity for lower ranked movements.
The 2000 Highway Capacity Manual provides skeleton coverage of modern
roundabouts, but does not define level-of-service at this time. Instead, the measure-of-effectiveness is the quotient of the volume to the capacity. A "modern roundabout" in the United States is a roundabout in which traffic inside the circle always has priority. Entering traffic is controlled by a yield sign.
Level-of-Service in Other Transportation Network Elements
Performance of other transportation network elements can also be communicated by LOS. Among them are:
* Two-lane roadways (uninterrupted flow)
* Multilane roadways (4 or more lanes) (uninterrupted flow)
* Open freeway segments
* Freeway entrances (merges), exits (diverges), and weaving lanes
* Bicycle facilities (measure-of-effectiveness: events per hour; events include meeting an oncoming bicyclist or overtaking a bicyclist traveling in the same direction)
* Pedestrian facilities (HCM measure-of-effectiveness: pedestrians per unit area)
The above grading refers to highways; however, some
professors in urban planningschools have proposed measurements of levels of service that take public transportationinto 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 densityin 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.
Level of Service in the U.K
The Level of Service measure is much more suited to American Roads than roads in Europe and the U.K, however the Highway Capacity Manual is used in the U.K. The technique does find its way into U.K textbooks, however in practice it is sparingly used in transportation analysis. The individual countries of the U.K have different bodies for each areas roads, and as a result detailed techniques and applications vary in
Scotland, Englandand Wales, however in general the practice is the same.
In the U.K rural and urban roads are in general much busier than in the U.S, and as such service levels tend to be to the higher end of the scale, especially in the peak commuting periods. It is acceptable for roads to operate at 85% capacity, which equates to D and E LOS.
In general the principle the U.K uses is to take the volume of traffic in one hour on the road and divide by the appropriate capacity of the road type to get a v/c rating. This v/c rating can be cross-referenced to the textbooks which publish tables of v/c ratings and their equivalent LOS ratings. The lack of definitive categories towards the D, E and F LOS ratings limit the use on U.K roads, as an D or E category on an urban road, would be acceptable in the U.K.
In certain circumstances the U.K shortens the LOS categories to just A-D. The first 2 categories indicate free-movement of traffic (i.e under 85% capacity), the C category indicates reaching capacity 85%-100%, whilst D indicates over capacity. Little reference to this can be found in textbooks and it may just be an 'unwritten engineering practice', agreed with certain authorities.
Level of Service in Australia
Refer Austroads Guide to Traffic Engineering Practice Part 2 for a good explanation.
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