qhdm vol1 part04 intersections and roundabouts octfinal

Volume 1

Part 4 Intersections and Roundabouts

VOLUME 1 PART 4 INTERSECTIONS AND ROUNDABOUTS

VOLUME 1

Disclaimer

The State of Qatar Ministry of Transport (MOT) provides access to the Qatar Highway Design Manual (QHDM) and Qatar Traffic Control Manual (QTCM) on the web and as hard copies as Version (1.0) of these manuals, without any minimum liability to MOT.

Under no circumstances does MOT warrant or certify the information to be free of errors or deficiencies of any kind.

The use of these manuals for any work does not relieve the user from exercising due diligence and sound engineering practice, nor does it entitle the user to claim or receive any kind of compensation for damages or loss that might be attributed to such use.

Any future changes and amendments will be made available on the MOT web site. Users of these manuals should check that they have the most current version.

Note: New findings, technologies, and topics related to transportation planning, design, operation, and maintenance will be used by MOT to update the manuals. Users are encouraged to provide feedback through the MOT website within a year of publishing the manuals, which will be reviewed, assessed, and possibly included in the next version.

Copyright © 2015. All rights reserved.


VOLUME 1

تنويه

ر دليل تصميم الطرق لدولة قطر (واصالت ملقامت وزارة ا ) ودليل Qatar Highway Design Manual ‐ QHDMي دولة قطر بتوفرنت وكنسخ مطبوعة باعتبارها اإلصدار رقم Qatar Traffic Control Manual ‐ QTCMقطر للتحكم املروري ( ى شبكة اإلن ) ع

ى ) من هذه األدلة1.0( .واصالتملوزارة اوذلك دون ادنى مسؤولية ع

ى إن ي هذين ملواصالت،اوزارة يجب التأكيد ع ى أن تكون املعلومات املتضمنة وتحت أي ظرف من الظروف، ال تج أو تتعهد أو ُتصادق عن خالية من أي نوع من األخطاء أو العيوب. الدليل

الهندسية السليمة، كما أنه ال يخول إن استخدام هذه األدلة ألي عمل ال يعفي املستخدم من إتباع العناية الواجبة أو الفائقة واملمارسة ى هذا االستخدام. للمستخدم املطالبة أو استالم أي نوع من التعويض عن األضرار أو الخسائر ال يمكن أن ُتعزى إ

ن التحقق بشكل متو ى املستخدم رنت الخاص بالوزارة. ويتوجب ع ى موقع اإلن رات او تعديالت متاحة ومتوفرة ع اصل بأن سوف تكون أي تغيم أحدث إصدار من هذه األدلة. لد

ن االعتبار االكتشافات الجديدة والتكنولوجيات الحديثة واصالت ملوزارة استقوم مالحظة: ن مع األخذ بع بمواصلة تحديث وتعديل ِكال الدليل واملرور. واملواضيع املُستجدة ال تتعلق بتخطيط وتصميم وتشغيل وصيانة النقل والطرق

ن ى تقديم املالحظات إن الوزارة ُتشجع املستخدم راحاتع ن، وذلك من واالق والتعليقات وردود األفعال، خالل سنة من اصدار ِكال الدليلراحاتخالل موقع الوزارة حيث سوف يتم مراجعة هذه املالحظات .ومن ثم تقييمها وإدراجها ضمن اإلصدار القادم من األدلة واالق


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Contents Page

Acronyms and Abbreviations ..................................................................................................... iii

1 Introduction ...................................................................................................................... 1 1.1 Overview ......................................................................................................................... 1 1.2 Intersection Type Selection Criteria................................................................................ 1 1.3 Intersection Objectives ................................................................................................... 2

2 Intersection Types and Their Applicability ........................................................................ 3 2.1 Types Related to Functional Classification ..................................................................... 3 2.2 Traffic Flow and Capacity ................................................................................................ 4

3 Intersection Selection ....................................................................................................... 7 3.1 Operational Quality ........................................................................................................ 7 3.2 Safety Performance ........................................................................................................ 8 3.3 Spatial Efficiency and Lifecycle Cost ............................................................................... 8 3.4 Decision Making in Intersection Type Selection ............................................................. 8

3.4.1 Rural Intersections .......................................................................................... 9 3.4.2 Urban Major Road Intersections ..................................................................... 9 3.4.3 Urban Local Road Intersections .................................................................... 10

3.5 Route Designation......................................................................................................... 10 3.6 Traffic Flows and Capacity ............................................................................................ 10 3.7 Local Conditions ............................................................................................................ 10 3.8 Overview of Operational and Design Trade-offs .......................................................... 10 3.9 Designation of Priority Intersections ............................................................................ 11

3.9.1 Needs for Traffic Signal Controlled Intersections ......................................... 12 3.10 Data and Analysis for Intersections .............................................................................. 13

4 Roundabout Type Selection ............................................................................................ 15 4.1 Route Designation......................................................................................................... 15 4.2 Traffic Flows and Capacity ............................................................................................ 15 4.3 Data and Analysis for Roundabouts .............................................................................. 16

5 High-volume At-grade Intersection Types ....................................................................... 19 5.1 Introduction .................................................................................................................. 19 5.2 Indirect Left-turn Intersections ..................................................................................... 19 5.3 “Superstreet” Intersections .......................................................................................... 21 5.4 Continuous-flow Intersections ...................................................................................... 21 5.5 Arterial-to-arterial Grade Separations .......................................................................... 24

References ............................................................................................................................... 25


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Tables

Table 2.1 Basic Forms of Intersection Types ....................................................................... 3

Table 2.2 Permitted Intersection Types on Urban and Rural Roads ................................... 4

Table 3.1 Summary of Basic Trade-offs Among Intersection Types .................................. 11

Figures

Figure 3.1 Progression of Decision Making in Intersection Type Selection .......................... 9

Figure 5.1 Indirect Left-turn Intersection with Median U-turn Crossover ......................... 20

Figure 5.2 “Superstreet” Intersection ................................................................................ 21

Figure 5.3 Continuous-flow Intersection ............................................................................ 23


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Acronyms and Abbreviations

FHWA Federal Highway Administration

m meter

MMUP Ministry of Municipality and Urban Planning

QHDM Qatar Highway Design Manual


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1 Introduction

1.1 Overview This part describes the initial process to select the appropriate form or concept for an intersection of two highways or roadways. When two highways intersect, the movement of traffic between and crossing them can be handled by at-grade solutions. These solutions include intersections, signalized and unsignalized, and roundabouts. Another type of solution, grade-separated interchange, is discussed in Part 9, Interchanges and Freeway or Motorway Corridors.

The selection of an appropriate intersection form is based on the following:

• Functional classification of the intersecting roads • The volume and pattern of traffic • The terrain and topography • Existing and planned land use, • The needs of nonmotorized users, such as pedestrians and cyclists

A brief methodology follows describing the fundamental steps to be undertaken when assessing intersection type and configuration options prior to the commencement of the detailed design.

1.2 Intersection Type Selection Criteria Intersections are a critical element of the highway transport system. They generally are the primary bottlenecks, producing delay to travelers. In urban areas in particular, most crashes are associated with intersections. Intersections are also central to the overall distribution of traffic through the highway network. Therefore, it is vital that appropriate intersection strategy and selection be considered early in road and corridor planning.

An intersection’s key purpose is to enable the safe and efficient transfer of traffic streams through the intersection from one road to another as well as across each road. The selection of the most appropriate type of intersection requires consideration of a number of factors, the most significant including road classification, traffic flows and capacities and safety. Decisions will represent trade-offs in operations, design costs and impacts.


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The following are key considerations in the selection of intersections:

• Intersections are by their nature points of conflict. Conflicts can translate into crashes. Design and operating strategies focus on reducing or eliminating conflicts, or reducing the severity of crashes.

• The capacity of the road network largely depends on the capacity of the intersections, particularly in urban areas. The overall traffic-carrying capability of an arterial highway with multiple lanes is defined not only by the number of lanes in the roadway segments, but also by the frequency, spacing, type and configuration of the intersections along the highway.

• In urban areas, intersections also represent points of conflict between pedestrians and motor vehicles.

Intersections may be between roads of similar functional classification, or roads of varying functional classification. Intersections involving roads of differing functional classification should be given particular consideration to provide an appropriate solution. Further guidance on applicable intersection types for each functional classification is provided in Section 2.1.

1.3 Intersection Objectives The following are typical objectives for intersections:

• Providing safe passage for all transportation modes and associated movements • Minimizing congestion and delay • Minimizing fuel consumption, air pollution and noise

Safety objectives should focus on minimizing the risk of severe crashes—crashes resulting in one or more fatalities or injuries. These objectives apply to all road and intersection users.

With regard to intersections, the term capacity, refers to the ability of the intersection to accommodate traffic demand from all approaches. Unlike a roadway segment in which traffic moves in one direction, an intersection provides for multiple, conflicting through and turning movements. Each movement will have both a design demand and a design and operational capacity. Designers need to be concerned not only with the overall capacity of the intersection but also with that of each individual component.

Most importantly, achieving all design objectives requires that trade-offs be made. Basic configurations and design features will produce conflicting outcomes. A solution that focuses solely on reducing crashes at the intersection will generally provide less capacity and operational quality than another solution with a different focus.


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2 Intersection Types and Their Applicability

2.1 Types Related to Functional Classification Various intersection types are available for use on the highway network. Table 2.1 details the basic forms to be considered.

Table 2.1 Basic Forms of Intersection Types

Intersection Type Description

Priority Intersection

At-grade intersection where the minor road terminates at the major road. Traffic control may be controlled by yield or stop signs and markings. Priority intersections may take various forms, depending on the number of links and their configuration. These are discussed in Part 6, Design for Priority Intersections. On divided highways, left-turn maneuvers are normally precluded by design and the intersection operates in a right-in, right-out mode.

Roundabout At-grade intersection incorporating a circulatory roadway around a central island. Intersection approaches operate in a “yield to entry” mode in which vehicles within the circulatory roadway have the right-of-way over those waiting on the approaches. Roundabouts treat all legs equally, versus favoring of one road over the other in a priority intersection. As such, they provide more capacity for minor road traffic, but in doing so impose delay and lower speeds on the major road. Roundabouts are discussed further in Part 7, Design for Roundabouts.

Signalized Intersection

An at-grade intersection where conflicting movements are separated over time by a signal control that allocates right-of-way in an alternating and regular pattern. In these intersections, it is the combination of roadway geometry and operation of the signal that establishes the capacity of the intersection and volume of traffic flow through it. Signalized intersections are further discussed in Part 8, Design for Signalized Intersections.

Grade-separated Interchange

Grade-separated interchanges have the greatest capacity of all intersection types. Conflicts are removed by physical separation of traffic, with one or more movements passing over or under the others. There are many configurations of interchanges, as discussed in Part 9, Interchanges and Freeway or Motorway Corridors.

In addition to the standard intersection types defined for intersecting roads, other facilities may be provided, including pedestrian crossings and median openings for


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U-turns. Information on pedestrian crossings including mid-block crossings are discussed in Part 19, Pedestrian, Bike, and Public Transportation.

The initial step in the intersection type selection process should involve reference to Table 2.2, which describes permitted intersection types for urban and rural roads according to functional classification of the major route. This table is based on guidance from the Transport Master Plan Qatar (Ministry of Municipality and Urban Planning [MMUP], 2007).

Table 2.2 Permitted Intersection Types on Urban and Rural Roads

Functional Classification of the Major Road


(Part 6) Roundabout

(Part 7)

Signalized Intersection

(Parts 8)

Grade-Separated

Interchange (Part 9)

URB

AN

Expressway X X X Major Arterial a X b Minor Arterial a X X Boulevard a X Collector Distributor a X X Major Collector X Minor Collector X Service Road X Local Road X

RURA

L

Freeway X X X Arterial a X

Collector X X Local Road X X

Notes: a. Right-in/ Right-out priority intersection only b. Grade separation provided only in exceptional circumstances

For many road types, there are multiple possible forms of intersection. For example, for a boulevard or collector (major or minor), a priority intersection, roundabout, or signal controlled intersection may be used. Conversely, for certain types of roads, by policy these are restricted to only specific forms. Urban expressways and rural freeways, as fully access-controlled facilities, are restricted to only grade-separated interchanges.

2.2 Traffic Flow and Capacity A critical factor in intersection type selection is the traffic flow and the predicted future traffic demand. Traffic flow for intersections includes all movements, both through and turning.

Before detailed evaluation can be made, it is important to obtain the best estimate of all the relevant traffic flows and turning movements for the intersection under design-year conditions. A preliminary analysis should be undertaken to establish the likely form of intersection considering the permitted types in Table 2.2.


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The composition and turning movements of traffic will influence the geometric layout adopted. Predicted future traffic flows are required for the following reasons:

• To enable the design to be tailored to provide sufficient capacity to meet the future traffic flow demands

• To size the intersection

• To enable a decision to be made to constrain the traffic flows at the given location for a particular reason

• To identify the need to allow for current or future intersections


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3 Intersection Selection

The selection of an appropriate intersection type involves application of the policies regarding functional classification and the specific attributes of the location being studied. These include the context as broadly defined and the design-year traffic demand for all users.

The general approach in selecting an intersection type reflects basic considerations of operational quality, access and accessibility, safety performance, spatial efficiency (right-of-way), and lifecycle cost effectiveness.

3.1 Operational Quality Minimizing delay and providing as high and uniform level of service is central to intersection type selection and its design. In selecting intersections the functional classification of the intersecting roadways is the initial consideration. Priority intersections are the backbone of the highway network. These are intended to provide necessary access and traffic distribution within the network while providing primacy of mobility to the higher-class roadway. For the highest roadway class, expressways and freeways, the requirement that intersecting movements occur through grade-separated interchanges enables the highest quality of flow possible for through traffic.

The inherent capacity of each intersection type differs. As traffic volumes and patterns increase, the appropriate intersection type changes.

• Priority intersections enable major road through traffic to proceed through the intersection with no delay. Delays to turning traffic increase with volume. Priority intersections on multilane roads maintain high quality of service by prohibiting conflicting left-turn movements.

• Roundabouts provide full turning and access capability. They result in speed reductions and a reduction of delays to through vehicles as well as turning vehicles. They have limited traffic-carrying capability; when their capacity is exceeded, higher-capacity solutions are required.

• Signalized intersections are the highest-capacity, at-grade solutions available. Their capacity is a function of the number of through and turning lanes provided and signal phasing and timing strategies employed.

• Grade-separated interchanges are used as discussed above for freeways and expressways. They may also apply in locations where the traffic demands exceed the capacity of a signalized intersection.


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Another important factor regarding operational quality involves the accessibility and mobility of pedestrians and bicyclists. Where these are significant demands, the intersection type should reflect community needs, and the design details and operation of the intersection should accommodate these users.

3.2 Safety Performance Each type of intersection—priority (yield and stop-controlled), roundabout, signalized and interchange—has inherently different risk profiles, as defined by the frequency, severity, and types of crashes that should be expected over time. In selecting an appropriate intersection type, the expected safety performance should be a consideration. Knowledge on the safety performance of each type is included throughout the Parts describing the intersections, and guidance is offered using this information.

3.3 Spatial Efficiency and Lifecycle Cost Each type of intersection requires a certain footprint, right-of-way, and lifecycle cost. Selection of the appropriate intersection type is thus fundamentally a design decision in which the above factors are weighed against the spatial needs and costs associated with its construction. For these reasons, designers may find that a different type of intersection is the best solution other than that normally used for the traffic conditions.

3.4 Decision Making in Intersection Type Selection Figure 3.1 shows the progression of thinking and decision making in intersection planning for both rural and urban conditions. As presented in the Qatar Highway Design Manual (QHDM), the hierarchy of intersection types can be generally characterized by location, which implies an expected design speed condition, and traffic demand.


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Figure 3.1 Progression of Decision Making in Intersection Type Selection

3.4.1 Rural Intersections Priority intersections predominate on rural roads. For the two-lane system, these provide access for turning movements. As traffic demands increase on the major road, the priority intersection type at some point can no longer process the demand and the safety performance may degrade. This leads to the need to implement a roundabout. For rural roads with typical traffic demands, roundabouts should be sufficient. In special cases (and specifically on rural freeways regardless of traffic demand) grade-separated interchanges may be used. Note that, because of the crash risks associated with high speed rear-end and angle crashes that can occur at traffic signals, signalized intersections on rural highways are not employed in Qatar.

3.4.2 Urban Major Road Intersections For urban arterials and collectors, the progression of type selection begins with priority intersections. Depending on the context (spatial availability, presence of pedestrians), as traffic increases, the next type of intersection considered may be either a roundabout or a signalized intersection. For very high-volume conditions the QHDM offers special intersection types and arterial-to-arterial grade separations designed for the urban environment. Expressway intersections should be interchanges.


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3.4.3 Urban Local Road Intersections For intersections on urban local roads, the operating speed of the road should be lower, and the design and operation of the intersections should place pedestrian safety of primary importance. The selection of the appropriate intersection type thus follows a progression that includes not only traffic demand but also pedestrian activity. Priority intersections may apply, but their design and operation may need to include stop control rather than yield and, in some cases, may include provision for pedestrian signals. All-stop intersections, not used for higher classification facilities, may be the most appropriate type because they support pedestrians crossing any intersection leg. Roundabouts do not apply for local road intersections (other than the possible use of mini-roundabouts for traffic calming or speed management). Local road intersections may need to be signalized to fully enable both vehicular turning traffic and pedestrians.

3.5 Route Designation The initial step should include determining the traffic flow and capacity of the intersection. The designer should confirm the functional classification of the intersecting roads and use that information to identify permitted intersection types as detailed in Table 2.2.

3.6 Traffic Flows and Capacity Consideration should be given to the expected traffic flows in the current year, the opening year, and the design year. In general, uncontrolled intersections are suitable only on local roads and where traffic flows are relatively low.

3.7 Local Conditions The context, as defined by land use, topography, and natural and manmade constraints, will influence the physical feasibility of a solution. Other important context features include the intersection location as part of the local road network. Proximity to adjacent intersections of different types will influence the selection of an appropriate type of intersection for a particular location. The existence of or need for access to adjoining properties is also a factor. Intersection selection clearly is based not only on traffic or functional classification, but also on the unique site context features.

3.8 Overview of Operational and Design Trade-offs Selection of the best intersection form will involve a site-specific analysis of the trade-offs. In most cases, the designer will have choices among the basic permitted forms. The trade-offs involve the transportation objectives of safety and mobility for the full range of users. They also involve the costs and impacts of each type of solution.

Table 3.1 is a qualitative summary of these trade-offs. It is intended to assist the designer in review and consideration of what are the most important objectives, what are the inherent attributes of each type, and what are the typical costs and challenges


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associated with implementation. Included in Table 3.1 for perspective is an assessment of grade-separated interchanges. Refer to Part 9, Interchanges and Freeway or Motorway Corridors.

Table 3.1 Summary of Basic Trade-offs Among Intersection Types

Intersection Types

Traffic Operations and Safety Costs and Adverse Effects of Implementation

Redu

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peed

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Tota

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Cost

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Visu

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Roundabout

Signal Controlled Intersection

Grade Separated Interchange

Notes:

Performs best Performs moderately successfully Performs worst

3.9 Designation of Priority Intersections Priority intersections are appropriate where traffic volumes are relatively light; for example, in many rural areas or where there are few conflicting turning movements, such as a right-in/right-out intersection. This intersection form favors the major road over the minor road, requires a minimum footprint, and has low cost of operation. This form has limitations in capacity associated with traffic movements on the minor road, especially when traffic flow on the major roadway is high. Left turns are prohibited, and the capacity of the right-turn discharge decreases as volume increases on the priority road.


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Typically, there are several options for priority intersections based on the geometric

requirements of the intersecting roads. These include simple intersections and

staggered T‐intersections. Further details are provided in Part 6, Design for Priority

Intersections.

3.9.1 Needs for Traffic Signal Controlled Intersections

Signalized intersections have more capacity than other at‐grade intersections. They

allow movements to be treated equally through the allocation of the circular green

signal. Signalized intersections provide the most positive protection for pedestrians

crossing at‐grade than the other two forms of intersections. Allocation of time to

movements and to pedestrians imposes delay on users. It also increases stopped and

idling time compared to other forms of intersections.

Traffic signal systems include signal installations, controllers, detectors, and other

hardware. These require continual maintenance. The cost of a signalized intersection

can be among the highest of at‐grade forms, depending on the number and

arrangement of lanes and the right‐of‐way needed.

The investigation of the need for a traffic control signal should include an analysis of

factors related to the existing operation and safety at the study location, the potential

to improve these conditions, and the criteria contained in the latest edition of the

Qatar Traffic Control Manual (MOT, 2015):

Criterion 1 – Intersection vehicular traffic flow volumes

Criterion 2 – Pedestrian flow volumes

Criterion 3 – School or civic amenity access

Criterion 4 – Coordination and management of traffic flow

Criterion 5 – Known crash locations

The criteria provide guidance on the types of situations where traffic control signals

are suitable. At least one of these criteria should be met before a traffic signal is

considered; however, satisfaction of a traffic signal criterion shall not in itself require

the installation of a traffic control signal.

For a traffic signal project to be considered, it is the responsibility of the design

engineer to consider the impact that traffic signals may have on the traffic locally and

the wider network, and act to provide the most efficient strategy to minimize network

delay. The engineer will complete a design and operational study that establishes the

benefits and costs.

Further details on the design of signal controlled intersections are provided in Part 8,

Design for Signalized Intersections.


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3.10 Data and Analysis for Intersections Classified traffic counts, including pedestrians and cyclists, for proposed operating conditions, including off-peak, should be conducted to assess the intersection operation for both opening and design years.

For signal controlled intersections, it is important that both peak and off-peak flows be considered, particularly where part-time signals are to be used. The intersection layout and arrangement should be designed to achieve an efficient layout and accommodate users and their desired paths or destinations.

Other basic data that are commonly required include the following:

• Topography at sites

• Adjacent land use, access points to properties, and special site constraints, such as the location of public utilities, trees, monuments, property boundaries, buildings, and drainage structures, including pipes

• Compilation and analysis of crash data for the most recent 5-year period. Five years is a typical period to use in the analysis of crashes; however, no fewer than 3 years should be used.

• Current traffic, including cyclists and pedestrians

• Special network functions, existing and proposed, such as freight routes and bus routes

• Values of economic factors, such as operating and delay costs, and rates for construction and maintenance work

• Property values and utility adjustment costs to be used in the analysis stage

• Budget limits

• Special constraints, such as political commitments, flood levels

• The predicted traffic flows and turning movements


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4 Roundabout Type Selection

4.1 Route Designation Roundabouts are intersections where traffic circulates counterclockwise around a central island; traffic entering a roundabout is required to yield to vehicles on the circulatory roadway. In Qatar, new roundabouts incorporate either one or two lanes on the circulatory roadway. Consequently, new roundabouts with three or more lanes on the circulatory roadway are not permitted.

Roundabouts can be useful where an intersection requires three or more approaches or where a transition of roadway cross sections is required. In general, roundabouts minimize delays for vehicles while maintaining safe passage of all road users through the intersection. Because vehicular traffic is required to slow down on the approach to a roundabout, these intersection types can also be a useful traffic-calming feature.

Location constraints are often a dominant factor, particularly in urban areas, and it may be found that provision of a roundabout is not appropriate due to difficulty in achieving a suitable geometric layout. Roundabouts also may not be suitable in areas that use urban traffic control, integrated demand management, or other circumstances where access control is required.

4.2 Traffic Flows and Capacity Consideration should be given to the expected traffic flows both in the opening or existing year and in the design year. In general, roundabouts can accommodate higher traffic volumes than priority intersections and lower traffic volumes than grade-separated interchanges or signalized intersections.

A primary value of the roundabout is its elimination of right-angle conflicts, which create the most severe crashes on higher-speed roads. For this reason, roundabouts are appropriate solutions on lower-volume, high-speed roads intersecting with other roads with similar speeds and volumes.

Roundabouts are intended to operate in a self-regulating manner. They perform satisfactorily when flows are balanced on each approach. A roundabout allows traffic from any approach to navigate through the circulatory roadway under the same basic yield-to-entry protocol. The operating costs of roundabouts are minimal, as there are no traffic signals to maintain. When a roundabout operates at traffic volumes well under its capacity, vehicles typically will be moving constantly, reducing braking, idling, and fuel emissions. However, this operating condition can be challenging for pedestrians desiring to cross one or more of the approaches.


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Roundabouts can take considerable space to construct. Access control is necessary on approaches. With these considerations and the concerns about pedestrians, the following conditions require special care in considering their use:

• There is an expected or forecast of significant growth in traffic through the roundabout.

• There is an expected or forecast of high or unbalanced demand on one or two of the approaches.

• Pedestrians are prevalent or expected to become prevalent based on future development.

• Patterns of traffic vary significantly throughout the day.

4.3 Data and Analysis for Roundabouts Classified traffic counts (including pedestrians and cyclists) for proposed operating conditions, including off-peak, should be undertaken to assess the intersection operation for both opening and design years.

The following data should be reviewed to determine the suitability of a roundabout for a particular situation:

• Whether the approach roads are single lane or multilane

• The speed limit on the approach roads

• The level of traffic flow

• The level of nonmotorized user flow

• Other constraints, such as right-of-way availability and access needs near one or more approaches

Part 7, Design for Roundabouts, provides for additional details regarding the design of roundabouts.

Other basic data that are commonly required include the following:

• Topography at sites

• Land use, access points to properties and special site constraints, such as the location of public utilities, trees, monuments, property boundaries, buildings, and drainage structures including pipes

• Compilation and analysis of crash data for the most recent 5-year period. Five years is a typical period to use in the analysis of crashes; however, no fewer than 3 years should be used.

• Special network functions, existing and proposed, such as freight routes and bus routes


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• Values of economic factors, such as operating and delay costs, and rates for construction and maintenance work

• Property values, utility adjustment costs to be used in the analysis stage

• Budget limits

• Special constraints, such as political commitments, flood levels


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5 High-volume At-grade Intersection Types

5.1 Introduction For most corridors and intersections, the solutions in previous chapters will be sufficient. However, as development and traffic volumes increase in Qatar and along certain highways, the need for creative solutions to intersection design and operations will emerge.

Agencies having to deal with very high traffic volumes on arterial streets have successfully tested and implemented unique design solutions for at-grade intersections. This section summarizes a selection of at-grade intersections that have potential applicability to the urban multilane arterials in Qatar.

A common theme among these solutions is addressing the dilemma created by the combination of high through volumes and high left-turning traffic volumes. In these situations, there is a limit to the capacity of a conventional signal controlled intersection. Increasing the number of left-turn lanes from two to three often produces little or no net benefit, as the space needed for the paths of three abreast turning requires operation of the intersection under highly inefficient “split phasing,” in which each approach has its own phase and simultaneous opposing movements are not possible.

Three special intersection forms can be implemented to address increased traffic volumes: indirect left-turn intersections, also known as Michigan left-turn intersections; “superstreet” intersections; and continuous-flow intersections.

5.2 Indirect Left-turn Intersections An indirect left-turn intersection is one in which left turns are prohibited. Drivers are directed by advance signing that their turn is to be made by turning right at the intersection, then proceeding down the crossroad to a median U-turn. From there, drivers proceed through the intersection again in the desired direction of travel. This treatment should apply to both left turns on the crossing road, thus necessitating U-turn roadways on both sides of the intersection.

By prohibiting left turns, the signal phase for it is eliminated, as is the need for storage of left turns on the approach. The operational trade-off is that left turns go out of direction and that traffic actually passes through the intersection twice—once as a


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right-turn movement and again as a through movement (Federal Highway Administration [FHWA], 2004).

Figure 5.1 illustrates an indirect left-turn intersection. This design is applicable for multilane arterials with sufficient median to allow the U-turn to be completed by a sufficiently large vehicle such as a single unit truck. Agencies that use this design often operate the U-turn as a yield or stop controlled intersection. The U-turn roadway should be 180 to 200 meters (m) or more away from the intersection. Special diagrammatic signing is used on the approach to the intersection to aid drivers.

The U-turn movement can also be signalized, with crossroad through traffic stopped for the U-turning traffic. If the median is wide enough, the U-turn movement can operate as a two-lane movement. Where signalization is used and the U-turn volumes are greater than 300 vehicles per hour, the separation of the U-turn from the intersection may need to increase to 200 m or more as indicated by a traffic analysis. Storage length requirements for the U-turn would be determined using the same design criteria applied to conventional left-turn storage.

Source: FHWA (2004).

Figure 5.1 Indirect Left-turn Intersection with Median U-turn Crossover


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5.3 “Superstreet” Intersections A “superstreet intersection” is very similar to the median U-turn above. As shown in Figure 5.2, movements for the crossroad turn right—both through and left-turning movements, are prohibited. Thus, the U-turn roadways are designed as two-lane U-turns and are signalized. The separation from the crossroad should be determined by the length of storage for the U-turn traffic using each crossover. In most cases, this storage will be 200 m or more as indicated by a traffic analysis. The operation of the single intersection that in conventional design would have four phases is translated into three intersections, each of which operates with only two phases. Note: Pedestrians can be routed through the middle of the intersection and cross during the signal phase carrying turning traffic from the major road into the crossroad.

Source: FHWA (2004).

Figure 5.2 “Superstreet” Intersection

5.4 Continuous-flow Intersections A continuous-flow intersection follows the same operational concept as that discussed in Section 5.3. A single, high-volume intersection requiring four phases for turning movements is translated into a local network of five coordinated two-phase intersections. In a continuous-flow intersection, the left turns are “transposed” or cross the opposing through traffic upstream of the major intersection. This movement is done using a signal. Once on the opposite side of the through traffic, drivers can continue with the turn to the crossroad during the same signal phase as the opposing through traffic.

This highly efficient concept can be applied to all four approaches, as shown on Figure 5.3, or to just one crossroad with conventional left turns operating on the other roadway. The concept also can be applied to high-volume, three-approach intersections.


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With a continuous-flow intersection, the spacing between the central intersection and approach crossovers should be at least 100 m, with additional storage as indicated by a traffic analysis. With storage length requirement for the crossover queues, the length of access control along each approach can be significant—as much as 200 m or more. The transposed left-turn movements should be physically separated from opposing traffic. The example continuous-flow intersection, shown on Figure 5.3, with full 3.65 m lane widths and separation, can require 130 m of total width (including right-turn lanes), possibly requiring the acquisition of additional right-of-way.


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5.5 Arterial-to-arterial Grade Separations For instances where a high-volume intersection, such as those presented in this section, is not possible, and traffic demand exceeds the capacity of a conventional signalized intersection, designers may need to consider an interchange between the two arterials. Part 9, Interchanges and Freeway or Motorway Corridors, describes basic arterial-to-arterial concepts that may have applicability in urban settings.


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References

Federal Highway Administration (FHWA). Signalized Intersections: Informational Guide. Report

No. FHWA‐HRT‐04‐091. U.S. Department of Transportation: Washington, DC, United States. 2004.

Ministry of Transport (MOT). Qatar Traffic Control Manual. Doha, Qatar. 2015.

Ministry of Municipality and Urban Planning (MMUP). Transport Master Plan Qatar. Doha, Qatar.

2007.

qhdm vol1 part04 intersections and roundabouts octfinal

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