9.traffic characteristics

TRAFFIC CHARACTERISTICS

INTRODUCTION

� A key component of the built environment is the transport system that serves the land uses contained within the urban fabric, whether they be residential, commercial, educational or other. commercial, educational or other.

� The road hierarchy has been used as a tool to assist in planning the interface between land use and the road system, and the appropriate linkage of roadways in the road system.

� Roadways serve a variety of functions, including

1. provision of direct access to properties, pedestrian and bicycle paths, bus routes and catering for through traffic that is not related to immediate land uses. uses.

2. Many roads serve more than one function and to varying degrees, but it is clear that the mixing of incompatible functions can lead to problems.

INTRODUCTION

� A road hierarchy is a means of defining each roadway in terms of its function such that appropriate objectives for that roadway can be set and appropriate design criteria can be implemented. design criteria can be implemented.

� These objectives and design criteria are aimed at

1. achieving an efficient road system whereby conflicts between the roadway and the adjacent land use are minimised.

2. appropriate level of interaction between the roadway and land use is permitted.

� The road hierarchy can then form the basis of ongoing planning and system management aimed at reducing the mixing of incompatible functions.

HIERARCHY OF URBAN ROADS

� The street hierarchy is an urban design technique for laying out road networks that exclude automobile through-traffic from developed areas.

� It establishes importance of each road type in the network topology i.e. connectivity � It establishes importance of each road type in the network topology i.e. connectivity of the nodes to each other.

� Street hierarchy restricts or eliminates direct connections between certain types of links, for example residential streets and arterial roads.

� Allows connections between similar order streets, e.g. arterial to arterial or between street types that are separated by one level in the hierarchy, e.g. arterial to highway and collector to arterial.

� Roads are classified into two broad categories i.e. Urban Roads and Non-urban roads. In India, urban roads are further classified as:

1. EXPRESSWAYS1. EXPRESSWAYS

2. ARTERIAL ROADS

3. SUB-ARTERIAL ROADS

4. COLLECTOR STREETS AND

5. LOCAL STREETS

ROAD HIERARCHY OBJECTIVES

� The key objective of a road hierarchy is to ensure the orderly grouping of roadways in a framework around which state and local governments can plan and implement various construction, maintenance, and management schemes and implement various construction, maintenance, and management schemes and projects.

� It should also assist local and state governments with the adoption of appropriate standards for roadway construction.

� A well formed road hierarchy will reduce overall impact of traffic by:-

1. concentrating longer distance flow onto routes in less sensitive locations;

2. ensuring land uses and activities that are incompatible with traffic flow are restricted from routes where traffic movement should predominate;

3. preserving areas where through traffic is discouraged;

4. ensuring activities most closely related to frontage development, including social interaction and parking, can be given more space within precincts where environmental and access functions should predominate.

OBJECTIVES OF ROAD HIERARCHY

� The road hierarchy principles will assist planning agencies with:-

1. orderly planning of heavy vehicle and dangerous goods routes;

planning and provision of public transport routes;2. planning and provision of public transport routes;

3. planning and provision of pedestrian and bicycle routes;

4. identifying the effects of development decisions in and on surrounding areas and roadways within the hierarchy;

5. development design that facilitates urban design principles such as accessibility, connectivity, efficiency, amenity and safety;

6. assigning control over access onto traffic carrying roads to ensure safe and efficient operation for traffic;efficient operation for traffic;

7. identifying treatments such as barriers, buffers and landscaping to preserve amenity for adjacent land uses.

CLASSIFICATION

OF

MAJOR ROADS

TYPOLOGY FEATURES EXAMPLES

National Expressways

1. Completely access controlled, tolled highways.2. Designed for high speed road travel.

• Mumbai PuneExpressway

National 1. Partially access controlled and tolled upgraded • Golden National Express Highways

1. Partially access controlled and tolled upgraded national highways.

2. Designed for high speed road travel.

• Golden Quadrilateral between Chennai-Bangalore

National Highways

1. Primary long-distance roadways. 2. Most of the National Highway are two-lane (one

NH8 Delhi to GurgaonHighways 2. Most of the National Highway are two-lane (one

in each direction) and undivided.3. Maintained by the GOI.4. Constitute 2% of the total road network of India,

but carry 40% of the total traffic.5. Highways which have heavy traffic intensity.6. Connect different State capitals, Major Ports,

large industrial areas and tourist centers .

Gurgaon

TYPOLOGY FEATURES

State Express Highways 1. State highways that have been upgraded to dual carriageways with improved road geometry and road safety features.

2. Tolled and partially access controlled state highways.

State Highways 1. Highways which link district headquarters, important towns and the National Highways in the State and neighboring States.

Major District Roads 1. Highways that connect production and marketing centers .

2. These centers in turn are connected with the National 2. These centers in turn are connected with the National Highways and State Highways.

Other District Roads 1. The Other District Roads connect Agricultural production centre's and marketing centre's with Taluk headquarters and other important roads nearby.

RELATIONSHIP BETWEEN HIERARCHY AND LANDUSES

� One of the key aims of the hierarchy is to optimise accessibility, connectivity, amenity and safety for all road users including motor vehicles, bicycles pedestrians, and public transport patrons. pedestrians, and public transport patrons.

� To do so, the relationship between hierarchy and the land uses it serves needs to be considered.

� The road hierarchy philosophy begins with consideration of the local area needs in what is termed a “specific area” or “environment cell”.

� A specific area is a part of the urban fabric that is contained within a “block” � A specific area is a part of the urban fabric that is contained within a “block” bordered by traffic carrying roads or other physical boundaries

1. Arterial roads carry through traffic external to the specific area, and sub arterial roads carry through traffic between multiple specific areas and the arterial roads.

1. Collector streets are located within the specific area, providing indirect and direct access for land uses within the specific area to the road network.

A. These streets should carry no traffic external to the specific area. A. These streets should carry no traffic external to the specific area.

B. The environmental cells within the specific area are bounded by the collector streets, and contain local streets with low speed environments and pedestrian priority.

C. Their function is to provide direct property access.

� Within environmental cells, considerations of amenity and environment dominate.

FOUR LEVEL ROAD HIERARCHY FRAMEWORK

� Four level road hierarchy framework are:

Level 1. Purpose relates to the primary objective of the element, whether Level 1. Purpose relates to the primary objective of the element, whether to carry through traffic or provide direct property access;

Level 2. Function relates to the relationship between the roadway and the land use it serves (i.e. how the roadway serves the land use);

Level 3. Management relates to the emplacement of policies to achieve the envisaged function based upon the attributes of the element and of the adjacent land uses; and

Level 4. Design relates to specification of the form of the element in Level 4. Design relates to specification of the form of the element in order to achieve its functional objectives.

� PURPOSE

� In general terms, traffic volume on a roadway is proportional to the number ofproperties served.

� Figure identifies the relationships between the importance of the accessfunction, and the traffic carrying function, and the number of propertiesserved.

� The greater number of properties served, the greater need there is for aroadway to serve a traffic carrying purpose.roadway to serve a traffic carrying purpose.

� In level 1 of the hierarchy we are able to define the basic purposes as follow:-

1. roads – to carry through traffic, serving a longer distance purpose;

2. streets – to provide access to properties and local areas.

CLASSIFICATION OF URBAN ROADS

� Figure 1: Speed vs accessibility


� Information on traffic characteristics is vital in selecting the appropriate geometric features of a roadway. Necessary traffic data includes Traffic Volume, Traffic Speed, and percentage of trucks or other large vehicles. Volume, Traffic Speed, and percentage of trucks or other large vehicles.

Traffic Volume

� Traffic volume is an important basis for determining what improvements, if any, are required on a highway or street facility.

� Traffic volumes may be expressed in terms of average daily traffic or design hourly volumes.

� These volumes may be used to calculate the service flow rate, � These volumes may be used to calculate the service flow rate, which is typically used for evaluations of geometric design alternatives.


� Average Daily Traffic.

1. Average daily traffic (ADT) represents the total traffic for a year divided by 365, or the average traffic volume per day. 365, or the average traffic volume per day.

2. AVERAGE DAILY TRAFFIC — The total traffic volume during a given time period (more than a day and less than a year) divided by the number of days in that time period.

3. Due to seasonal, weekly, daily, or hourly variations, ADT is generally undesirable as a basis for design, particularly for high-volume facilities.

4. ADT should only be used as a design basis for low and moderate volume facilities, where more than two lanes unquestionably are not justified.

� Design Hourly Volume.

1. The design hourly volume (DHV) is usually the 30th highest hourly volume for the design year, commonly 20 years from the time of construction completion.

2. For situations involving high seasonal fluctuations in ADT, some adjustment of DHV may be appropriate.


� Design Hourly Volume.

3. For two-lane rural highways, the DHV is the total traffic in both directions oftravel.

4. On highways with more than two lanes (or on two-lane roads where importantintersections are encountered or where additional lanes are to be providedlater), knowledge of the directional distribution of traffic during the designhour (DDHV) is essential for design.

5. DHV and DDHV may be determined by the application of conversion factorsto ADT.

� DIRECTIONAL DESIGN HOUR VOLUME —� DIRECTIONAL DESIGN HOUR VOLUME —

1. The traffic volume expected to use a highway segment during the 30th highesthour of the design year in the peak direction.


� Computation of DHV and DDHV.

1. The percent of ADT occurring in the design hour (K) may be used to convertADT to DHV as follows:ADT to DHV as follows:

A. DHV = (ADT)(K)

2. The percentage of the design hourly volume that is in the predominantdirection of travel (D) and K are both considered in converting ADT to DDHVas shown in the following equation:

A. DDHV = (ADT)(K)(D)

� DESIGN HOUR — The 30th highest hour of the design year.� DESIGN HOUR — The 30th highest hour of the design year.

� DESIGN HOUR FACTOR


� Directional Distribution (D).

1. The percentage of total, two-way peak hour traffic that occurs in thepeak direction.peak direction.

2. D30 — The proportion of traffic in the 30th highest hour of the design yeartravelling in the peak direction.

3. D100 — The proportion of traffic in the 100th highest hour of the designyear travelling in the peak direction. D100 is often used in calculating thelevel of service for a roadway.

4. D200 — The proportion of traffic in the 200th highest hour of the designyear traveling in the peak direction.

5. DF — Directional distribution factor for ESALD equation. Use 1.0 if one-5. DF — Directional distribution factor for ESALD equation. Use 1.0 if one-way traffic is counted or 0.5 for two-way.

� Traffic tends to be more equally divided by direction near the centre of anurban area or on loop facilities.

� For other facilities, D factors of 60 to 70 percent frequently occur.


� K Factors.

1. The proportion of Annual Average Daily Traffic (AADT) occurring in an hour.

2. Proportion of 24-hour volume occurring during the design hour for a givenlocation or area.

3. K is the percentage of ADT representing the 30th highest hourly volume in thedesign year.

4. For typical main rural highways, K-factors generally range from 12 to 18percent. For urban facilities, K- factors are typically somewhat lower, rangingfrom 8 to 12 percent.

5. K30 — The proportion of Annual Average Daily Traffic (AADT) occurring during the 30th highest hour of the design year. Commonly known as the Design Hour the 30th highest hour of the design year. Commonly known as the Design Hour Factor.

6. K100 — The proportion of Annual Average Daily Traffic (AADT) occurring during the 100th highest hour of the design year. Commonly known as the Planning Analysis Hour Factor.

7. K200 — The proportion of Annual Average Daily Traffic (AADT) occurring during the 200th highest hour of the design year.


� Projected Traffic Volumes.

Projected traffic volumes are provided by the Transportation Planning and 1. Projected traffic volumes are provided by the Transportation Planning and Programming Division upon request and serve as a basis for design of proposed improvements.

2. For high-volume facilities, a tabulation showing traffic converted to DHV or DDHV will be provided by that Division if specifically requested. Generally, however, projected traffic volume is expressed as ADT with K and D factors provided.

� NOTE: If the directional ADT is known for only one direction, total ADT may be computed by multiplying the directional ADT by two for most cases.


� Service Flow Rate.

1. A facility should be designed to provide sufficient capacity to accommodatethe design traffic volumes (ADT, DHV, DDHV).the design traffic volumes (ADT, DHV, DDHV).

2. The necessary capacity of a roadway is initially based on a set of “idealconditions.” These conditions are then adjusted for the “actual conditions”that are predicted to exist on the roadway section.

3. This adjusted capacity is termed service flow rate (SF) and is defined as ameasure of the maximum flow rate under prevailing conditions. Adjustingfor prevailing conditions involves adjusting for variations in the followingfactors:

A. lane widthA. lane width

B. lateral clearances

C. free-flow speed

D. terrain

E. distribution of vehicle type.


� Service Flow Rate.

1. Facilities are classified into two categories of flow: uninterrupted and interrupted. interrupted.

2. Uninterrupted-flow facilities have no fixed elements, such as traffic signals, that are external to the traffic stream and might interrupt the traffic flow.

3. Traffic flow conditions result from the interactions among vehicles in the traffic stream and between vehicles and the geometric and environmental characteristics of the roadway.

4. Interrupted-flow facilities have controlled and uncontrolled access points that can interrupt the traffic flow.

5. These access points include traffic signals, stop signs, yield signs, and other types of control that stop traffic periodically (or slow it significantly), irrespective of the amount of traffic.

� The service flow rates generally are based on a 15-min period. Typically, the hourly service flow rate is defined as four times the peak 15-min volume.


Traffic Speed

� Traffic speed is influenced by volume, capacity, design, weather, traffic control devices, posted speed limit, and individual driver preference.devices, posted speed limit, and individual driver preference.

� For design purposes, the following definitions apply:

1. Low-speed is 45 mph [70 km/h] and below .

2. High-speed is 50 mph [80 km/h] and above .

� Design Speed.

1. Design speed is defined as the highest continuous speed at which individual vehicles can travel with safety . This speed is defined as that speed which is greater than the speed of 85% of drivers.

Design speed is a selected speed used to determine the various geometric 2. Design speed is a selected speed used to determine the various geometric design features of the roadway.

3. It is important to design facilities with all elements in balance, consistent with an appropriate design speed.

4. Design elements such as sight distance, vertical and horizontal alignment, lane and shoulder widths, roadside clearances, super elevation, etc., are influenced by design speed.


4. Selection of design speed for a given roadway is influenced by the character ofterrain, economic considerations, extent of roadside development (i.e., urban orrural), and highway type.rural), and highway type.

5. For example, the design speed chosen would usually be less for rough terrain, orfor an urban facility with frequent points of access, as opposed to a rural highwayon level terrain.

6. Choice should be influenced by the expectations of drivers, which are closelyrelated to traffic volume conditions, potential traffic conflicts, and topographicfeatures.

Posted Speed.� Posted Speed.

1. Posted speed refers to the maximum speed limit posted on a section of highway.

2. Posted speed should be based primarily upon the 85th percentile speed whenadequate speed samples can be secured.

3. Speed zoning guidelines permit consideration of other factors such as roadsidedevelopment, road and shoulder surface characteristics, public input, andpedestrian and bicycle activity.


CAPACITY

� Is the maximum number of vehicles which have a reasonable expectation of passing over a section of a lane or a roadway during a given time period under passing over a section of a lane or a roadway during a given time period under prevailing roadway and traffic conditions.

OR

The maximum rate of flow at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a specified time period under prevailing roadway, traffic and control conditions; usually expressed as vehicles per hour or persons per hour.

� Capacity encompasses the relationship between highway characteristics and conditions, traffic composition and flow patterns, and the relative degree of congestion at various traffic volumes throughout the range from light volumes to those equalling the capacity of the facility as defined above.


CAPACITY

� Highway capacity information serves three general purposes:

For transportation planning studies to assess the adequacy or sufficiency of1. For transportation planning studies to assess the adequacy or sufficiency ofexisting highway networks to current traffic demand, and to estimate when,in time, projected traffic demand, may exceed the capacity of the existinghighway network or may cause undesirable congestion on the highwaysystem.

2. For identifying and analyzing bottleneck locations (both existing andpotential), and for the evaluation of traffic operational improvementprojects on the highway network.

For highway design purposes.3. For highway design purposes.


CAPACITY

� Another important concept in understanding the capacity of roadway is theLevel of Service.Level of Service.

� The level of service concept places various traffic flow conditions into 6 levels ofservice. These levels of service, designated A through F, from best to worst,cover the entire range of traffic operations that may occur.

� The factors that may be considered in evaluating level of service include thefollowing:

1. Speed and travel time.

2. Traffic interruptions or restrictions.2. Traffic interruptions or restrictions.

3. Freedom to manoeuvre.

4. Safety.

5. Driving comfort and convenience.

6. Economy

� In practical approach to identifying the level of service, travel time and the ratioof demand volume to capacity are commonly used

CLASSIFICATION OF URBAN ROADS

ROAD CLASSIFICATION

Roads classification criteria

� Apart from the classification given by the different plans, roads were also classifiedbased on some other criteria. They are given in detail below.

1. Based on usage

A. This classification is based on whether the roads can be used during differentA. This classification is based on whether the roads can be used during differentseasons of the year.

B. All-weather roads: Those roads which are negotiable during all weathers,except at major river crossings where interruption of traffic is permissible upto a certain extent are called all weather roads.

C. Fair-weather roads: Roads which are negotiable only during fair weatherare called fair weather roads.

2. Based on carriage way

A. This classification is based on the type of the carriage way or the roadpavement.

B. Paved roads with hard surface : If they are provided with a hardpavement course such roads are called paved roads.(eg: stones, Water boundmacadam (WBM), Bituminous macadam (BM), concrete roads)

C. Unpaved roads: Roads which are not provided with a hard course of atleast a WBM layer they are called unpaved roads. Thus earth and gravel roadscome under this category.

ROAD CLASSIFICATION

� The roads can be classified in many ways.

� The classification based on speed and accessibility is the most generic one.

� As the accessibility of road increases, the speed reduces. Accordingly, the roads canbe classified as follows in the order of increased accessibility and reduced speeds.be classified as follows in the order of increased accessibility and reduced speeds.

� Freeways:

1. Freeways are access-controlled divided highways.

2. Most freeways are four lanes, two lanes each direction, but many freewayswiden to incorporate more lanes as they enter urban areas.

3. Access is controlled through the use of interchanges, and the type ofinterchange depends upon the kind of intersecting road way (rural roads,another freeway etc.)another freeway etc.)

� Expressways:

1. They are superior type of highways and are designed for high speeds (120km/hr), high traffic volume and safety.

2. They are generally provided with grade separations at intersections.

3. Parking, loading and unloading of goods and pedestrian traffic is not allowedon expressways.

ROAD CLASSIFICATION

� Highways:

1. They represent the superior type of roads in the country.

2. A long-distance, medium speed vehicular corridor that traverses opencountry.

3. A highway should be relatively free of intersections, driveways and adjacentbuildings; otherwise it becomes a strip, which interferes with traffic flow.

4. Highways are of two types - rural highways and urban highways.

5. Rural highways are those passing through rural areas (villages) and urbanhighways are those passing through large cities and towns, ie. urban areas.

� Arterials:

1. It is a general term denoting a street primarily meant for through traffic1. It is a general term denoting a street primarily meant for through trafficusually on a continuous route.

2. They are generally divided highways with fully or partially controlled access.

3. Parking, loading and unloading activities are usually restricted andregulated.

4. Pedestrians are allowed to cross only at intersections/designated pedestriancrossings.

� Local streets :1. A local street is the one which is primarily intended for access to residence,

business or abutting property.It does not normally carry large volume of traffic and also it allows unrestricted

ROAD CLASSIFICATION

2. It does not normally carry large volume of traffic and also it allows unrestrictedparking and pedestrian movements.

3. Provide access to property.4. Less than 2,500 vehicles per day.5. Low traffic speed.6. No bus routes.7. Sidewalks on at least one side of road.8. Low priority for winter maintenance

� Collector streets: 1. These are streets intended for collecting and distributing traffic to and from local

streets and also for providing access to arterial streets. 2. Normally full access is provided on these streets . There are few parking

restrictions except during peak hours.

TYPOLOGY FEATURES

Expressway 1. High speed signal-free arterial roads which are either

completely elevated or has grade separated intersections.

Arterial Roads 1. Extensions of National Highways into the city centre. Arterial Roads 1. Extensions of National Highways into the city centre. These corridors usually carry the bulk of commuting traffic to and from the city and from one end to the other, like ring roads.

Primary Collector or Sub-arterial Roads

1. Road corridors that carries a huge volume of traffic between arterial roads and has a high density of public bus transport services.

Secondary Collector 1. Collector roads that carry lower volume of traffic and has Secondary Collector Roads

1. Collector roads that carry lower volume of traffic and has a lower public bus transport service.

Main Roads and feeder roads

1. Main roads that collects local traffic from residential areas onto bigger roads.

2. These roads also acts as shortcuts and is used by non local traffic.

3. There is no regular bus services on these roads.

TYPOLOGY RIGHT OF WAY NUMBER OF LANES

Expressway 61 metres on the periphery of

the town. Otherwise 49m.

One on either side of the central median. To start with, and two on either side later on, as needed.

Arterial Roads 25m to 30m Two lanes to start with and four Arterial Roads 25m to 30m Two lanes to start with and four later on as needed.

Primary Collector or Sub-arterial Roads

18m to 25m Two lanes to start with and morewhen required.

Secondary Collector Roads

14m to 18m Two lanes plus two parking lanes.Collector Roads

Main Roads and feeder roads

9m to 14m . Two lanes

Cul –de – sac 8m to 9m Two lanes.

TRAFFIC INTERSECTIONS/ INTERCHANGES

� Intersection is an area shared by two or more roads. This area is designated for the vehicles to turn to different directions to reach their desired destinations.

TRAFFIC INTERSECTIONS

� Traffic intersections are complex locations on any highway.

� Drivers have to make split second decision at an intersection by considering his route, intersection geometry, speed and direction of other vehicles etc.

� Overall traffic flow depends on the performance of the intersections. It also affects the capacity of the road.

� Conflicts at an intersection are different for different types of intersection.

� Therefore, both from the accident perspective and the capacity perspective, the study of intersections is very important for the traffic engineers especially in the case of urban scenario.

�Auxiliary lanes at intersections serve a wide range of purposes including space for

deceleration and acceleration, bus stops, increased capacity through an intersection, and

storage for turning vehicles.

�The capacity of a signalized intersection may be increased by adding an auxiliary lane

TRAFFIC INTERSECTIONS

�The capacity of a signalized intersection may be increased by adding an auxiliary lane

to accommodate through traffic.

� MAJOR STREET: Intersecting

street with greater volume, larger

cross-section, higher functional

class.

� MINOR STREET: Intersecting

street with less traffic volume,street with less traffic volume,

smaller cross-section.

TRAFFIC INTERSECTION CONTROLS

� The essence of the intersection control is to resolve the conflicts at theintersection for the safe and efficient movement of both vehicular traffic andpedestrians.pedestrians.

� Two methods of intersection controls are there:

1. Time Sharing: At the same instance of time, traffic moving in onedirection is allowed to move while the other moving in different directionis stopped. For e.g.: this achieved with the help of traffic signals.

2. Space Sharing: Space sharing intersections where vehicles weave,merge and separate e.g.. Traffic circles

� The type of intersection control to be adopted depends on the traffic volume,� The type of intersection control to be adopted depends on the traffic volume,road geometry, cost involved, importance of the road etc.

� The control of an intersection can be exercised at different levels. They canbe either passive control, semi control, or active control.

PASSIVE CONTROL

� When the volume of traffic is less, no explicit control is required. Here theroad users are required to obey the basic rules of the road.

� Passive control like traffic signs, road markings etc. are used to complementPassive control like traffic signs, road markings etc. are used to complementthe intersection control.

� Some of the intersection control that are classified under passive control areas follows:

1. No Control: If the traffic coming to an intersection is low, then byapplying the basic rules of the road like driver on the left side of the roadmust yield and that through movements will have priority than turningmovements. The driver is expected to obey these basic rules of the road.

2. Traffic signs:With the help of warning signs, guide signs etc. it is able to2. Traffic signs:With the help of warning signs, guide signs etc. it is able toprovide some level of control at an intersection. Give way control, two-waystop control, and all-way stop control are some examples.

PASSIVE CONTROL

A. GIVE WAY control requires the driver in the minor road to slow down to a minimum speed and allow the vehicle on the major road to proceed.

B. Two Way Stop control requires the vehicle drivers on the minor streets Two Way Stop control requires the vehicle drivers on the minor streets should see that the conflicts are avoided.

C. All-way Stop control used when it is difficult to differentiate between the major and minor roads in an intersection. STOP sign is placed on all the approaches to the intersection and the driver on all the approaches are required to stop the vehicle.

3. Traffic signs plus marking: In addition to the traffic signs, road markings also complement the traffic control at intersections. Some of the examples include stop line marking, yield lines, arrow markingexamples include stop line marking, yield lines, arrow marking

SEMI CONTROL

� In semi control or partial control, the drivers are gently guided toavoid conflicts. Channelization and traffic rotaries are two examples ofthis.this.

1. Channelization:

A. The traffic is separated to flow through definite paths by raisinga portion of the road in the middle called as islands defined byroad markings.

B. The conflicts in traffic movements are reduced to a great extentin such a case. In channelized intersections, the traffic isdirected to flow through different channels and this physicalseparation is made possible with the help of some barriers in theseparation is made possible with the help of some barriers in theroad like traffic islands, road markings etc.

SEMI CONTROL

2. Traffic Rotaries:

A. It is a form of intersection control in which the traffic is made to flowalong one direction around a traffic island.along one direction around a traffic island.

B. The essential principle of this control is to convert all the severe conflictslike through and right turn conflicts into milder conflicts like merging,weaving and diverging.

C. It is a form of `at-grade' intersection laid out for the movement of trafficsuch that no through conflicts are there.

D. Free-left turn is permitted where as through traffic and right-turn trafficis forced to move around the central island in a clock-wise direction in anorderly manner.orderly manner.

SEMI CONTROL

2. Traffic Rotaries:

E. Traffic operations at a rotary are three;diverging, merging and weaving. All thediverging, merging and weaving. All theother conflicts are converted into thesethree less severe conflicts.

1.1. DivergingDiverging: It is a traffic operation whenthe vehicles moving in one direction isseparated into different streamsaccording to their destinations.

2.2. MergingMerging: Merging is the opposite ofdiverging.diverging.

Merging is referred to as the process of joining the traffic comingfrom different approaches and going to a common destination into asingle stream.

3.3. WeavingWeaving:: Weaving is the combined movement of both merging anddiverging movements in the same direction.

ACTIVE CONTROL

� Active control implies that the road user will be forced to follow the pathsuggested by the traffic control agencies.

� Traffic signals and grade separated intersections come under thisclassification.

1. Traffic Signals:

A. Control using traffic signal is based on time sharing approach.

B. At a given time, with the help of appropriate signals, certain trafficmovements are restricted where as certain other movements arepermitted to pass through the intersection.permitted to pass through the intersection.

C. When the vehicles traversing the intersection is very large, then thecontrol is done with the help of signals.

ACTIVE CONTROL

2. Grade Separated Intersections:

A. The intersections are of two types. They are at-grade intersections andgrade-separated intersections.grade-separated intersections.

B. In at-grade intersections, all roadways join or cross at the same verticallevel. Grade separated intersections allows the traffic to cross at differentvertical levels.

C. They are usually constructed on high speed facilities like expressways,freeways etc.

D. These type of intersection increases the road capacity because vehiclescan flow with high speed and accident potential is also reduced due tovertical separation of traffic.vertical separation of traffic.

TYPES OF INTERSECTIONS AND CONFIGURATIONS

� Intersections can be categorized into four major types:

1. Simple Intersections

Flared Intersections2. Flared Intersections

3. Channelized Intersections

4. Roundabouts

� SIMPLE INTERSECTIONS

1. Simple intersections maintain the street’s typical cross-section and number of lanes throughout the intersection, on both the major and minor streets.

2. Simple intersections are best-suited to locations where auxiliary (turning) 2. Simple intersections are best-suited to locations where auxiliary (turning) lanes are not needed to achieve the desired level of-service, or are infeasible due to nearby constraints.

3. Generally, simple intersections provide the minimum crossing distances for pedestrians and are common in low-volume locations.


FLARED INTERSECTIONS

� Flared intersections expand the cross-section of the street (main, cross or both).

� The flaring is done to accommodate a left-turn lane, so that left turning bicycles� The flaring is done to accommodate a left-turn lane, so that left turning bicyclesand motor vehicles are removed from the through-traffic stream to increasecapacity at high-volume locations, and safety on higher speed streets.

� Intersections may be flared to accommodate an additional through lane as well.

� Intersection approachescan be flared slightly, notenough for additionalapproach lanes but simplyto ease the vehicle turningto ease the vehicle turningmovement approachingor departing theintersection.


FLARED INTERSECTIONS

� This type of flaring has benefits to bicycle and motor vehicular flow since higherspeed turning movements at the intersection are possible and encroachment byspeed turning movements at the intersection are possible and encroachment bylarger turning vehicles into other vehicle paths is reduced.

� However, adding flare to an intersection increases the pedestrian crossingdistance and time.

CHANNELIZED INTERSECTIONS

� Channelized intersections use pavement markings or raised islands to designate the intended vehicle paths.

� The most frequent use is for right turns, particularly when accompanied by an � The most frequent use is for right turns, particularly when accompanied by an auxiliary right-turn lane.

� At intersections located on a curve, divisional islands can help direct drivers to and through the intersection.

� At large intersections, short median islands can be used effectively for pedestrian refuge.


CHANNELIZED INTERSECTIONS

� Channelized intersections are usuallylarge and, therefore, require longlarge and, therefore, require longpedestrian crosswalks.

� Channelization islands can effectivelyreduce the crosswalk distance in whichpedestrians are exposed to movingmotor vehicles.

� The design of channelized intersections needs to ensure that the needs ofpedestrians are considered. Allowing wheelchair users the same safe harbour asother pedestrians on channelization islands.other pedestrians on channelization islands.


ROUNDABOUTS

� The roundabout is a channelizedintersection with one-way trafficintersection with one-way trafficflow circulating around a centralisland.

� All traffic—through as well asturning—enters this one-way flow.

� Although usually circular in shape,the central island of a roundaboutcan be oval or irregularly shaped.

Roundabouts can be appropriate design alternative to both stop controlled and� Roundabouts can be appropriate design alternative to both stop controlled andsignal-controlled intersections, as they have fewer conflict points thantraditional intersections.

� At intersections of two-lane streets, roundabouts can usually function with asingle circulating lane, making it possible to fit them into most settings.


ROUNDABOUTS

� Size – Single lane roundabouts have an outside diameter between 80 and 140feet.feet.

� Speed – The small diameter of roundabouts limits circulating vehicle speedsto 10 to 25 miles per hour.

� Capacity – The slower circulating speeds at roundabouts allow enteringvehicles to accept smaller gaps in the circulating traffic flow, meaning moregaps are available, increasing the volume of traffic processed.

� Safety – The slower speeds at roundabouts not only reduce the severity ofcrashes, but minimizes the total number of all crashes.

� Roundabouts are also considered as traffic-calming devices in some locationssince all traffic is slowed to the design speed of the one-way circulatingroadway.

GRADE SEPARATED INTERSECTIONS

� Grade-separated intersections are provided to separate the traffc in the vertical grade.

� But the traffic need not be those pertaining to road only. When a railway line � But the traffic need not be those pertaining to road only. When a railway line crosses a road, then also grade separators are used.

� Different types of grade-separators are flyovers and interchange.

� Flyovers itself are subdivided into overpass and underpass.

1. When two roads cross at a point, if the road having major traffic is elevated to a higher grade for further movement of traffic, then such structures are called overpass.

2. If the major road is depressed to a lower level to cross another by means of 2. If the major road is depressed to a lower level to cross another by means of an under bridge or tunnel, it is called under-pass.

� Interchange is a system where traffic between two or more roadways flows at different levels in the grade separated junctions.

� Common types of interchange include trumpet interchange, diamond interchange , and cloverleaf interchange.


� TRUMPET INTERCHANGE:

1. Trumpet interchange is a popular form ofthree leg interchange.three leg interchange.

2. If one of the legs of the interchange meets ahighway at some angle but does not cross it,then the interchange is called trumpetinterchange.

� DIAMOND INTERCHANGE:

1. Diamond interchange is a form of four-leg1. Diamond interchange is a form of four-leginterchange found in the urban locationswhere major and minor roads crosses.

2. The important feature of this interchange isthat it can be designed even if the major roadis relatively narrow.


� CLOVER LEAF INTERCHANGE:

1. It is also a four leg interchange and isused when two highways of highused when two highways of highvolume and speed intersect each otherwith considerable turning movements.

2. The main advantage of cloverleafintersection is that it provides completeseparation of traffic, as well as highspeed at intersections can be achieved.Disadvantage is that large area of landis required.is required.

9.traffic characteristics

Documents

hierarchy of urban roads

formed road hierarchy

street hierarchy

road type

road networks

efficient road system

nonurban roads

traffic flow