Level of service (LOS) is a measure used by traffic engineers to determine the effectiveness of elements of transportation infrastructure. LOS is most commonly used to analyze highways by categorizing traffic flow with corresponding safe driving conditions. The concept has also been applied to intersections, transit, and water supply.
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The following section pertains to only North American highway LOS standards and it uses the letters A through F, with A being the best and F being the worst.
Level-of-Service A describes free-flow operations. Traffic flows at or above the posted speed limit and all motorists have complete mobility between lanes. The average spacing between vehicles is about 550 ft(167m) or 27 car lengths. Motorist have a high level of physical and psychological comfort. The effects of incidents or point breakdowns are easily absorbed. An example of LOS A occurs late at night in urban areas, frequently in rural areas, and generally in car advertisements.
Level-of-Service B describes reasonable free-flow operations. Free-flow (LOS A) speeds are maintained, maneuverability within the traffic stream is slightly restricted. The lowest average vehicle spacing is about 330 ft(100m) or 16 car lengths. Motorist still have a high level of physical and psychological comfort.
Level-of-Service C describes at or near free-flow operations. Ability to maneuver through lanes is noticeably restricted and lane changes require more driver awareness. Minimum vehicle spacing is about 220 ft(67m) or 11 car lengths. At LOS C most experienced drivers are comfortable, roads remain safely below but efficiently close to capacity, and posted speed is maintained. Minor incidents may still have no effect but localized service will have noticeable effects and traffic delays will form behind the incident. This is the targeted LOS for some urban and most rural highways.
Level-of-Service D describes decreasing free-flow levels. Speeds slightly decrease as the traffic volume slightly increase. Freedom to maneuver within the traffic stream is much more limited and driver comfort levels decrease. Vehicles are spaced about 160 ft(50m) or 8 car lengths. Minor incidents are expected to create delays. Example of 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. It is a common goal for urban streets during peak hours, as attaining LOS C would require a prohibitive cost and societal impact in bypass roads and lane additions.
Level-of-Service E describes operations at capacity. Flow becomes irregular and speed varies rapidly because there are virtually no usable gaps to maneuver in the traffic stream and speeds rarely reach the posted limit. Vehicle spacing is about 6 car lengths, however speeds are still at or above 50 mi/h(80 km/h). Any disruption to traffic flow, such as merging ramp traffic or lane changes, will create a shock wave affecting traffic upstream. Any incident will create serious delays. Driver's level of comfort become poor.[1] LOS E is a common standard in larger urban areas, where some roadway congestion is inevitable.
Level-of-Service F describes a breakdown in vehicular flow. Flow is forced; every vehicle moves in lockstep with the vehicle in front of it, with frequent slowing required. 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
C=Stable flow
D=Approaching unstable flow
E=Unstable flow
F=Forced or breakdown flow
The Highway Capacity Manual defines 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.
| 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 |
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 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.
Performance of other transportation network elements can also be communicated by LOS. Among them are:
The level of service concept was first developed for highways in an era of rapid expansion in the use and availability of the private motor car. The primary concern was congestion, and it was commonly held that only the rapid expansion of the freeway network would keep congestion in check.
Since then, 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.
While it may be tempting to aim for an "A" Level of Service, this is unrealistic in urban areas. Urban areas more typically adopt standards varying between "C" and "E", depending on the area's size and characteristics, while "F" is sometimes allowed in areas with improved pedestrian, bicycle, or transit alternatives. More stringent Level of Service standards (particularly in urban areas) tend to necessitate the widening of roads to accommodate development, thus discouraging use by these alternatives. Because of this, some planners recommend increasing population density in towns, narrowing streets, managing car use in some areas, providing sidewalks and safe pedestrian and bicycle facilities, 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. It should be noted that this standard is not considered a good measure of pedestrian facilities by the planning or engineering professions, because it rates undesirable (and hence unused) sidewalks with a Level of Service "A", while pedestrians tend to prefer active, interesting sidewalks, where people prefer to walk (but rate a worse Level of Service on this scale). To rectify this and other issues, The National Cooperative Highway Research Program (NCHRP) is conducting a project to enhance methods to determine Levels of Service for automobile, transit, bicycle, and pedestrian modes on urban streets, with particular consideration to intermodal interactions.
Walking
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Queueing
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Stairs
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The A to F scale described above deals only with delays and service reliability. These delays are typically caused by congestion, breakdowns or infrequent service. It assumes there is a service in place that people can use. It also implies that poor levels of service can be solved by increased capacity such as additional lanes, overcoming bottlenecks, and in the case of transit, more buses or trains. It does not deal for instance with cases where there is no bridge across a river, no bus or train services, no sidewalks, or no bike-lanes.
An expanded level of service might look like: 0 - No Service exists. Latent demand may exist. 1 - Service is poor, unsafe or discouraging. Demand is suppressed below socially desirable levels. A-F - As per existing level-of-service scale. G - Further expansion of capacity is limited. H - No expansion is possible. Radical or innovative solutions are required
The Level of Service measure is much more suited to American Roads than roads in Europe and the UK, however the Highway Capacity Manual is used in the UK. The technique does find its way into UK textbooks, however in practice it is sparingly used in transportation analysis. The individual countries of the UK have different bodies for each areas roads, and as a result detailed techniques and applications vary in Scotland, England and Wales, however in general the practice is the same.
In the UK 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 UK 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 UK roads, as an D or E category on an urban road, would be acceptable in the UK.
In certain circumstances the UK 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.
In Australia levels of service are an integral component of Asset Management Plans. In that context they can be defined as the service quality for a given activity. Levels of service are often documented as a commitment to carryout a given action or actions within a specified time frame in response to an event or asset condition data.[2]
Refer Austroads Guide to Traffic Engineering Practice Part 2 for a good explanation.