What Does Highway Level Of Service Depend On

In this article we will hash out nigh:- 1. Definition of Highway Capacity 2. Importance of the Concept of Highway Capacity iii. Factors Affecting Capacity and Level of Service 4. Traffic Capacity Formula.

Definition of Highway Capacity :

Highway capacity is associated with traffic book and traffic density.

Traffic book is the number of vehicles passing a given point on a roadway during a specified time menses. This is unremarkably expressed as vehicles per hour.

Traffic density is defined equally the number of vehicles occupying a unit length of a lane of a roadway at a given instant of fourth dimension. This is ordinarily expressed in vehicles per kilometre.

It is piece of cake 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 fourth dimension period under prevailing roadway, traffic and control conditions. (While traffic volume represents the actual charge per unit of flow and responds to variations in traffic demand, capacity indicates the maximum rate of menstruation when exposed to a certain level of service characteristics or a number of prevailing roadway and traffic atmospheric condition).

The Highway Capacity Transmission (1950) divers the following 3 types of highway capacity (O. M Normann):

Basic Capacity:

Basic capacity is the maximum number of vehicles that tin laissez passer a given point on a lane or a roadway during one 60 minutes, under the ideal roadway and traffic conditions that can possibly be attained.

Possible Capacity:

Possible capacity is the maximum number of vehicles that tin can pass a given point on a lane or roadway during i hour, under the prevailing roadway and traffic weather condition.

Practical Capacity:

Practical capacity is the maximum number of vehicles that can pass a given signal in a lane or roadway during i 60 minutes, when traffic density is not so great as to cause unreasonable delay, hazard or restriction to the driver's freedom to manoeuvre nether prevailing roadway and traffic conditions. This is usually considered to be the 'pattern capacity'.

It is observed that the term 'capacity' used in the revised Highway Capacity Transmission is the aforementioned as the 'possible capacity' defined above.

The 'basic chapters' of the original Highway Manual (1950) has been replaced with 'capacity under platonic weather condition'. 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 bodily field observations; alternatively, it tin can be calculated from prophylactic stopping sight distance considerations involving total reaction fourth dimension and braking altitude, 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 enquiry in this management and published the Highway Capacity Transmission in 1950; this was later revised in 1965, 1985 and 2000 with further improvements combining experimental and mathematical models of traffic menstruum.

Importance of the Concept of Highway Chapters :

The concept of highway capacity is of import for the following reasons:

1. The capacity of a highway should be adequate to serve the needs of the projected traffic.

ii. The course 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 exist finer planned based on the studies of highway chapters.

4. The adequacy of the existing highway network for the existing traffic volume can be assessed past capacity studies; transportation planning can be washed effectively using this information.

Passenger Car Unit (PCU) :

Highway facilities in Bharat are used past different classes of vehicles such equally cars, buses, trucks, vans, auto-rickshaws, motor-cycles, bicycles, bullock-carts then on. The characteristics of this heterogeneous or mixed traffic flow are complex compared to homogenous traffic consisting of passenger cars merely.

In guild to facilitate the estimation of traffic volume and traffic chapters while dealing with mixed traffic conditions, information technology 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 machine. The mutual unit, therefore, is the passenger car unit (PCU).

If the addition of one detail vehicle per 60 minutes of a certain class affects the traffic flow to the same extent every bit the add-on of x passenger cars, that particular vehicle is considered equivalent to x PCU.

The important factors that bear upon 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 performance.

4. Regulation and command measures of traffic.

five. Road surroundings (urban, rural, signalized etc.)

6. Climatic and atmospheric condition conditions.

The IRC guidelines for capacity recommended the following PCU equivalent for dissimilar types of vehicles in India for rural conditions:

The latest IRC guidelines in this regard are given beneath:

The Highway Capacity Manual (HCM), USA, defines the ideal conditions for uninterrupted flow every bit follows:

i. Traffic flow, gratuitous from interference of vehicles and pedestrians from the side

2. In-stream catamenia of passenger cars

3. Traffic lanes of adequate width and shoulders with no lateral obstructions within ane.8m from the edge of the carriageway

iv. 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. (Co-ordinate to HCM, the theoretical capacity under ideal conditions ranges from 2800 PCU/hour (full 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, United states of america):

When a road is conveying a traffic book equal to its capacity under ideal roadway and traffic conditions, the service offered by the road to the route user becomes poor and the driving speed drops leading to an increment in travel time every bit there is no overtaking opportunity.

Thus, the service offered by a roadway to the road user can vary depending on the traffic book. Level of Service denotes the level of facility that tin can be derived from a road nether dissimilar traffic volumes and operating conditions. The concept of level of service is defined as a qualitative measure of the operating weather condition in a traffic stream and the perception of these weather condition by drivers and passengers

The following factors are considered in evaluating the level of service:

(i) Operating speed and travel time/journey time.

(ii) Traffic interruptions and restrictions – speed changes and delays involved.

(iii) Freedom to manoeuvre to maintain the operating speed.

(iv) Driving comfort

(5) Economy, with minimum operating cost of vehicle.

Although it is desirable to consider all these factors, it is difficult to contain 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 book 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 beneath (Level A represents the highest and level F the lowest):

Level A – Free period, with low volumes and high speeds low traffic density speed controlled by driver's desires and concrete roadway weather. There is no restriction on maneuverability due to the presence of other vehicles.

Level B – Zone of stable flow, operating speeds kickoff to be restricted by traffic atmospheric condition. At that place is reasonable freedom to select speed and lane. The lower limit of this level (lowest speed and highest volume) is associated with service book 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 all the same obtained, with service volumes suitable for urban design do.

Level D – Approaches unstable period, the operating speed is tolerable, just considerably affected by changes in operating weather. Fluctuations in volume and restrictions to menstruum may cause substantial drops in operating speed. Drivers accept petty freedom to manoeuvre. Comfort and convenience are low but tin exist tolerated for short periods.

Level E – This level cannot be described by speed solitary. It is as well determined by volumes at or near highway capacity. Typical speeds are 50km/h. Menstruation 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 chapters. Queues of vehicles piling upwardly from a restriction downstream serve as a storage expanse during acme hour. Substantial reduction in speeds and stoppages may occur for brusk or long duration considering of downstream congestion. In the farthermost, both speed and book tin drops to zero.

The traffic volume that can be served at each level of service is known every bit the 'service volume'. Subsequently selecting a level of service for pattern purposes, the corresponding service volume logically becomes the pattern book or design capacity. If this volume is exceeded, the operating weather will fall below the level of service selected.

The highway capacity transmission makes no recommendations regarding the level of service to exist selected for the blueprint of different types of highways. The choice is left to the designer to select an advisable level of service based on economy and need.

Fig. 4.49 shows the typical relationship betwixt operating speed and volume/capacity ratio. The zones where the different levels of service occur are shown in this effigy, which pertains to a multi-lane highway.

[Annotation: This is like to speed-volume part. Initially, the book increases every bit the speed increases up to a sure 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 exist maximum (Fig. four.16)].

Factors Affecting Capacity and Level of Service:

The factors affecting chapters and level of service fall nether 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 bear upon capacity and level of service. Even road intersections bear upon the capacity.

Lane width – A width of iii.65 m is considered ideal for shine catamenia. Smaller widths than this will reduce the chapters up to 25 per cent. Distance from the edge of the carriageway to an obstruction is also an important factor which tin can bear on capacity.

Lateral clearance – Lateral obstructions may be abutments such equally sign posts, lite poles and parked cars; if these are located closer than ane.83 m from the edge of a traffic lane, it reduces the capacity.

Shoulders – They aid to maintain traffic flow. Paved shoulders of one.2 m width increase the effective width of the lane past 0.3m.

Horizontal alignment – Sharp curves and restrictive sight distances are likewise factors which tend to reduce lane chapters. The highway capacity manual contains tables to account for the influence of these factors on capacity.

Slope – Gradients bear upon the speed of the vehicles, especially trucks, and have an adversely bear on on the capacity.

Pavement surface condition – A deteriorated pavement surface condition is known to adversely affect the capacity only the Manual states that adequate data are non bachelor to develop adjustment factors to consider this.

Intersections-at-grade – These touch on the chapters significantly.

(b) Traffic Atmospheric condition:

The capacity and level of service are affected by the composition of different types of vehicles in the stream, variation of traffic flow, traffic pause, 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.

Chapters 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 one, 05,000 PCU/twenty-four hours 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 admission, no standing vehicles, and very niggling cross traffic; for other restrictive conditions, the capacity values will exist reduced significantly.)