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The final version, published as the Highway Capacity Manual, Sixth Edition: A Guide for Multimodal Mobility Analysis, or HCM 2016, or HCM6, was released in October 2016 and is available from TRB. The sixth edition incorporates the latest research on highway capacity, quality of service, active traffic and demand management, and travel time.
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In this article we will discuss about:- 1. Definition of Highway Capacity 2. Importance of the Concept of Highway Capacity 3. Factors Affecting Capacity and Level of Service 4. Traffic Capacity Formula.
Definition of Highway Capacity:
Highway capacity is associated with traffic volume and traffic density.
Traffic volume is the number of vehicles passing a given point on a roadway during a specified time period. This is usually expressed as vehicles per hour.
Traffic density is defined as the number of vehicles occupying a unit length of a lane of a roadway at a given instant of time. This is usually expressed in vehicles per kilometre.
It is easy to understand that traffic volume is the product of traffic density and speed.
Capacity is the maximum traffic flow that can be accommodated in a highway facility during a given time period under prevailing roadway, traffic and control conditions. (While traffic volume represents the actual rate of flow and responds to variations in traffic demand, capacity indicates the maximum rate of flow when exposed to a certain level of service characteristics or a number of prevailing roadway and traffic conditions).
The Highway Capacity Manual (1950) defined the following three types of highway capacity (O. K Normann):
Basic capacity is the maximum number of vehicles that can pass a given point on a lane or a roadway during one hour, under the ideal roadway and traffic conditions that can possibly be attained.
Possible capacity is the maximum number of vehicles that can pass a given point on a lane or roadway during one hour, under the prevailing roadway and traffic conditions.
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Practical capacity is the maximum number of vehicles that can pass a given point in a lane or roadway during one hour, when traffic density is not so great as to cause unreasonable delay, hazard or restriction to the driver’s freedom to manoeuvre under prevailing roadway and traffic conditions. This is usually considered to be the ‘design capacity’.
It is observed that the term ‘capacity’ used in the revised Highway Capacity Manual is the same as the ‘possible capacity’ defined above.
The ‘basic capacity’ of the original Highway Manual (1950) has been replaced with ‘capacity under ideal conditions’. The original ‘practical capacity’ has been replaced by several ‘service volumes’ related to a group of desirable operating conditions collectively termed as ‘level of service’.
The value of S is obtained from actual field observations; alternatively, it can be calculated from safe stopping sight distance considerations involving total reaction time and braking distance, and the length of the vehicles, L, in metres –
Realizing the inadequacy of this approach, the Highway Research Board Committee on Highway Capacity (U.S), headed by O.K. Normann, conducted research in this direction and published the Highway Capacity Manual in 1950; this was later revised in 1965, 1985 and 2000 with further improvements combining experimental and mathematical models of traffic flow.
Importance of the Concept of Highway Capacity:
The concept of highway capacity is important for the following reasons:
1. The capacity of a highway should be adequate to serve the needs of the projected traffic.
2. The class of highway, lane width, number of lanes and intersections are dependent on capacity.
3. Improvements on geometric elements, traffic control devices and traffic management measures can be effectively planned based on the studies of highway capacity.
4. The adequacy of the existing highway network for the existing traffic volume can be assessed by capacity studies; transportation planning can be done effectively using this information.
Passenger Car Unit (PCU):
Highway facilities in India are used by different classes of vehicles such as cars, buses, trucks, vans, auto-rickshaws, motor-cycles, bicycles, bullock-carts and so on. The characteristics of this heterogeneous or mixed traffic flow are complex compared to homogenous traffic consisting of passenger cars only.
In order to facilitate the estimation of traffic volume and traffic capacity while dealing with mixed traffic conditions, it is imperative that a common standard vehicle is chosen and all other types are converted into this class of vehicle; the standard vehicle chosen for this purpose is the passenger car. The common unit, therefore, is the passenger car unit (PCU).
If the addition of one particular vehicle per hour of a certain class affects the traffic flow to the same extent as the addition of x passenger cars, that particular vehicle is considered equivalent to x PCU.
The important factors that affect the PCU-value of any type of vehicle are:
1. Size – the length and width of the vehicle
2. Speed under the prevailing roadway and traffic
3. Clearances – transverse and longitudinal for ensuring safe traffic operation.
4. Regulation and control measures of traffic.
5. Road environment (urban, rural, signalized etc.)
6. Climatic and weather conditions.
The IRC guidelines for capacity recommended the following PCU equivalent for different types of vehicles in India for rural conditions:
The latest IRC guidelines in this regard are given below:
The Highway Capacity Manual (HCM), USA, defines the ideal conditions for uninterrupted flow as follows:
1. Traffic flow, free from interference of vehicles and pedestrians from the side
2. In-stream flow of passenger cars
3. Traffic lanes of adequate width and shoulders with no lateral obstructions within 1.8m from the edge of the carriageway
4. Horizontal and vertical alignment suitable for an average speed of 100km/h on multi-lane highways in rural areas.
These ideal conditions are rarely met with on actual roads; hence, the theoretical capacities are never realised. (According to HCM, the theoretical capacity under ideal conditions ranges from 2800 PCU/hour (total in both directions for a two-way, two-lane highway) to 2200 per lane with free speeds of 100km/h for multi-lane highways.
Level of Service Concept (HCM, USA):
When a road is carrying a traffic volume equal to its capacity under ideal roadway and traffic conditions, the service offered by the road to the road user becomes poor and the driving speed drops leading to an increase in travel time as there is no overtaking opportunity.
Thus, the service offered by a roadway to the road user can vary depending on the traffic volume. Level of Service denotes the level of facility that can be derived from a road under different traffic volumes and operating conditions. The concept of level of service is defined as a qualitative measure of the operating conditions in a traffic stream and the perception of these conditions by drivers and passengers
The following factors are considered in evaluating the level of service:
(ii) Traffic interruptions and restrictions – speed changes and delays involved.
(iii) Freedom to manoeuvre to maintain the operating speed.
(v) Economy, with minimum operating cost of vehicle.
Although it is desirable to consider all these factors, it is difficult to incorporate all in the absence of data. In view of this, the highway capacity manual utilises only two factors – the travel speed and the ratio of the service volume to capacity, depending upon the nature of the problem. While operating speed is used in rural areas to evaluate the level of service, in urban areas, the average overall travel speed is taken into account.
The operating conditions for the six levels of service selected by HCM are given below (Level A represents the highest and level F the lowest):
Level A – Free flow, with low volumes and high speeds low traffic density speed controlled by driver’s desires and physical roadway conditions. There is no restriction on maneuverability due to the presence of other vehicles.
Level B – Zone of stable flow, operating speeds beginning to be restricted by traffic conditions. There is reasonable freedom to select speed and lane. The lower limit of this level (lowest speed and highest volume) is associated with service volume used in the design of rural highways.
Level C – Still in the zone of stable flow, but speeds and maneuverability more closely controlled by higher volumes. Drivers are restricted regarding speeds lane changes and overtaking manoeuvres. A relatively satisfactory operating speed is still obtained, with service volumes suitable for urban design practice.
Level D – Approaches unstable flow, the operating speed is tolerable, but considerably affected by changes in operating conditions. Fluctuations in volume and restrictions to flow may cause substantial drops in operating speed. Drivers have little freedom to manoeuvre. Comfort and convenience are low but can be tolerated for short periods.
Level E – This level cannot be described by speed alone. It is also determined by volumes at or near highway capacity. Typical speeds are 50km/h. Flow is unstable and there may be stoppages of short durations.
Level F – At this level, there are forced operations at low speeds resulting in volumes below capacity. Queues of vehicles piling up from a restriction downstream serve as a storage area during peak hour. Substantial reduction in speeds and stoppages may occur for short or long duration because of downstream congestion. In the extreme, both speed and volume can drops to zero.
The traffic volume that can be served at each level of service is known as the ‘service volume’. After selecting a level of service for design purposes, the corresponding service volume logically becomes the design volume or design capacity. If this volume is exceeded, the operating conditions will fall below the level of service selected.
The highway capacity manual makes no recommendations regarding the level of service to be selected for the design of different types of highways. The choice is left to the designer to select an appropriate level of service based on economy and need.
Fig. 4.49 shows the typical relationship between operating speed and volume/capacity ratio. The zones where the different levels of service occur are shown in this figure, which pertains to a multi-lane highway.
[Note: This is similar to speed-volume function. Initially, the volume increases as the speed increases up to a certain limit; beyond this, as speed increases the driver’s freedom to manoeuvre decreases and the volume tends to decrease. Thus, there will be an optimum operating speed at which the volume will be maximum (Fig. 4.16)].
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Factors Affecting Capacity and Level of Service:
The factors affecting capacity and level of service fall under two heads:
(a) Roadway Conditions:
Restrictive physical and geometric elements such as lane-width, lateral clearance, and horizontal alignment of road shoulders, gradients and pavement surface condition are some factors which affect capacity and level of service. Even road intersections affect the capacity.
Lane width – A width of 3.65 m is considered ideal for smooth flow. Smaller widths than this will reduce the capacity up to 25 per cent. Distance from the edge of the carriageway to an obstruction is also an important factor which can affect capacity.
Lateral clearance – Lateral obstructions may be abutments such as sign posts, light poles and parked cars; if these are located closer than 1.83 m from the edge of a traffic lane, it reduces the capacity.
Shoulders – They help to maintain traffic flow. Paved shoulders of 1.2 m width increase the effective width of the lane by 0.3m.
Horizontal alignment – Sharp curves and restrictive sight distances are also factors which tend to reduce lane capacity. The highway capacity manual contains tables to account for the influence of these factors on capacity.
Gradient – Gradients affect the speed of the vehicles, especially trucks, and have an adversely impact on the capacity.
Pavement surface condition – A deteriorated pavement surface condition is known to adversely affect the capacity but the Manual states that adequate data are not available to develop adjustment factors to consider this.
Intersections-at-grade – These affect the capacity significantly.
(b) Traffic Conditions:
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The capacity and level of service are affected by the composition of different types of vehicles in the stream, variation of traffic flow, traffic interruption, and lane distribution. The number of traffic lanes, vehicular and driver characteristics, and one- or two-way traffic movements have a direct bearing on the lane or highway capacity.
Capacity of Rural Roads:
The latest IRC recommendations for design service volumes are given below:
For four-lane divided roads, the design service volumes range from 47,000 to 1, 05,000 PCU/day depending upon the terrain, shoulder-type and the level of service (B or C)
Capacity of Urban Roads:
Capacity values for urban roads (between intersections suggested by the IRC are given below:
(These are for no frontage access, no standing vehicles, and very little cross traffic; for other restrictive conditions, the capacity values will be reduced significantly.)