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Chapter 31
Channelization
31.1 Introduction
One of the most effective and efficient methods of controlling the traffic on a highway is the adoption of high intersection geometric design standards. Channelization is an integral part of at grade intersections and is used to separate turning movements from through movements where this is considered advisable and hence helps reduce the intensity and frequency of loss of life and property due to accidents to a large extent. Proper channelization increases capac- ity, improves safety, provides maximum convenience, and instils driver confidence. Improper channelization has the opposite effect and may be worse than none at all. Over channelization should be avoided because it could create confusion and worsen operations.
31.2 Definitions and Important Terms
- Channelization - It is the separation or regulation of conflicting traffic movements into definite paths of travel by traffic islands or pavement marking to facilitate the safe and orderly movements of both vehicles and pedestrians.
- Conflict - It is defined as the demand for the same highway space by two or more users of the highway. Conflicts are classified into mainly three types:
(a) Crossing conflicts (b) Diverging conflicts (c) Merging conflicts
- Angle of Intersection - The angle of intersection is that formed by the centerlines of the intersecting streets. Where the angle of intersection departs significantly (more than approximately 20o) from right angles, the intersection is referred to as a skewed
Minor Angle of Intersection
Major Leg
Leg
Figure 31:1: Angle of Intersection
intersection. Fig. 31:1 shows the angle made between the centre lines of the major and minor legs.
- Refuge Areas - The area which is used to give refuge to the pedestrians crossing a street (the open area between two medians) is known as a refuge area.
31.3 Objectives of Channelization
The use of channelization is often creative and innovative, providing for vehicle path separation and distinct and thus in general making traffic flow safer, smoother, simpler and efficient. The main objectives of channelization can be summarized as follows:
- Separation of maneuver areas: The drivers should be presented with only one decision at a time to reduce confusion and the influence of operations caused due to the overlapping of maneuver areas.
- Reduce excessively large paved areas: The spread of the paved area can be consider- ably reduced by the construction of raised islands and medians where these are considered safe and necessary.
- Control of maneuver angle:The intensity of accidents can be reduced to a large extent by providing small angles for merging, diverging and weaving (at low relative speeds) and approximately right angles for crossing (at high relative speeds). The maneuver angle can be easily controlled by constructing islands of appropriate shapes and sizes.
- Favor predominant turning movements: Channelization is also directed for giving preference to turning movements at an intersection where the proportion of such traffic is high.
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2
4 3
6
1
Figure 31:3: Illustration of T-intersection channelization, (b)Intersection with right-turn and passing lane(Source: [4])
Figure 31:4: Illustration of T-intersection channelization, (c)Fully channelized intersec- tion(Source: [4])
Figure 31:5: Conflict area in all paved intersection (Source: [4])
31.4 Design Principles
Design of a channelized intersection usually involves the following significant controls: the type of design vehicle, the cross sections on the crossroads, the projected traffic volumes in relation to capacity, the number of pedestrians, the speed of vehicles, and the type and location of traffic control devices. Furthermore, the physical controls such as right-of-way and terrain have an effect on the extent of channelization that is economically feasible. The degree to which each of these principles applies will depend upon the features mentioned above. While a principle may be modified in its application to a particular site, disregard of these may result in a hazardous design. The principles may be summarized as follows:
- Reduction of the Area of Conflict: The impact area is decreased when channelization is provided, and hence the probability of conflicts is also reduced. The figure below further clarifies the statement. Fig. 31:5 shows the conflict area in a Y-intersection without channelization and Fig. 31:6 shows the reduced conflict area in the same intersection after providing medians.
- Merging traffic streams at small angles: Merging at small angles permits the flow of traffic streams with minimum speed differentials. Hence, the gap acceptance time is also small in such cases. The merging of roadways should be done as shown below in Fig. 31:7.
3.5m
3.5m
4m
6m
6m 4m
Figure 31:9: Reduction of speed by funneling (Source: [4])
Figure 31:10: Refuge area for protecting crossing or turning traffic (Source: [4])
the intersection. Fig. 31:9 shows the funneling technique used for reduction of speed.
- Protection for turning vehicles/crossing conflicting traffic streams: Provision of a refuge area between the two opposing streams allows the driver of a crossing vehicle to select a safe gap in one stream at a time and also provides a safer crossing maneuver. Fig. 31:10further clarifies the above statement.
- Discourage prohibited turns by island placement and shape: Undesirable and prohibited turns can be discouraged by the proper selection of shape and location of the islands. Fig. 31:11 shows how prohibited turns can be discouraged by proper shaping and placement of islands.
- Providing locations of traffic control devices: Channelization may provide locations for the installation of essential traffic control devices, such as stop and directional signs, signals etc. Fig. 31:12 shows how channelizing devices can also be used for locating traffic
ONE WAY
Figure 31:11: Properly placed islands discourage prohibited movements (Source: [4])
^ ^
Figure 31:12: Location of signal posts on medians at intersections (Source: [4])
control devices.
31.5 Channelizing devices
A channelizing device can be defined as any structure which helps in providing channelization. These can be wide raised medians, non-traversable road islands, traversable raised curbs or even flush channelizing devices. A brief description of the various devices which are used for the purpose of channelization are given in the following sections.
- Wide Raised Medians In this form of channelizing device, a raised wide separator is constructed between the two opposing lanes and the space on the separator (median) is used either for planting some trees and/or for providing space for traffic signs etc. Fig. 31:13 shows a typical wide raised median on a freeway. A median varying between 1.2 m and 30 m in width may be employed. The higher values of width are adopted on freeways, where sufficient space is available for the construction of these. In addition, a well-landscaped wide median
Figure 31:14: Non-traversable Raised Island (source: [13])
Figure 31:15: Traversable Raised Curb System (without vertical panels) (Source: [15])
Figure 31:16: Traversable Raised Curb System (with vertical panels) (Source: [16])
Figure 31:17: Flush island providing channelization objectives (Source: [2])
- Flush Channelization In this type of channelization, a variety of treatments, including raising them above the pavement just slightly (2 to 5 cm); the application of pavement markings and other types of contrasting surfaces etc are possible. These may also be unpaved where they are formed by the pavement edges of existing roadways. In areas where snow plowing may be necessary, flush islands are the preferred design. Fig. 31:17 below shows how flush islands can also be used for achieving channelizing objectives. The area seen flushed with the road surface in Fig. 31:17 is the flush island.
31.6 Traffic Islands
A principle concern in channelization is the design of the islands. An island is a defined area between traffic lanes for control of vehicle movements. Within an intersection area, a median or an outer separation is considered to be an island. It may range from an area delineated by barrier curbs to a pavement area marked by paint.
31.6.1 Classification of Islands
Traffic islands usually serve more than one function, but may be generally classified in three separate types:
- Channelizing Islands - These are designed to control and direct traffic movement, usually turning. Channelizing islands are are shown in Fig. 31:18.
- Divisional Islands - These are designed to divide opposing or same direction traffic
Figure 31:20: Refuge Islands (Source: [17])
traffic into orderly movements. The total design of traffic islands can be studied in three steps:
- Selection of appropriate island type (barrier, mountable, painted or flush): The site and traffic conditions in each intersection are different and hence the island type suitable for each requires separate attention. The traffic island selected may vary from barrier type islands to flush islands marked on the roadway surface.
- Determination of shape and size of islands: The shape of the island and its size in an intersection depends on the geometry and space availability at the same. A proper shape and size of the island (in case of raised islands) must be selected so that it is able to both channelize the traffic and not pose any type of hazard.
- Location relative to adjacent traffic lanes: The islands must be offset from the roadway by some distance to remove the risk of a vehicle dashing against the same. The width of offset is maximum at the entry of the island and decreases gradually as one moves towards the end of it.
31.6.3 Guidelines for selection of island type
As mentioned earlier, each intersection has a unique geometry and flow values, and hence needs special attention as far as the use of channelization devices are concerned. The main factors affecting the selection of the island type are:
- Traffic characteristics at the intersection
- Cost considerations, and
- Maintenance needs
The raised islands and flush channelization are dealt with in details in the following sections.
Flush Channelization
Flush Channelization is usually appropriate in the following conditions:
- On high speed rural highways to separate turning lanes.
- In constrained locations, i.e. the locations where vehicle path definition is desired but space for raised islands not available.
- For separating opposing traffic streams of low speed streets.
- In areas where frequent removal of snowfall is required, i.e. in places of high snow fall.
- It can also be used as a temporary channelization either during construction or to test traffic operations prior to the actual installation of raised islands.
However, the main demerits of this type of channelization are :
- It is not effective in prohibiting or preventing traffic movements.
- It is also not appropriate for islands intended to serve as pedestrian refuge.
Raised Islands
The locations where the construction of raised islands assumes importance are:
- The primary function of the channelizing device is shielding pedestrians or to provide refuge to pedestrians crossing a street.
- Also, the primary/secondary function is locating traffic signals or other fixed objects.
- Intension is to prohibit or prevent certain traffic movements.
- To separate high volume opposing traffic flows.
- The raised islands are also particularly important at intersections with unusual geometry i.e. skewed intersections.
A comparison between the usefulness and the operating conditions of the two types of chan- nelization is presented in Table. 31:1.
Table 31:2: Recommended Island Sizes (Source: [9]) Location of Intersection Size(Sq.metres) Minimum Desired Urban 4.65 7 Rural and High Speed urban/Suburban 7 9.
Direction of Traffic
Direction of Traffic
Direction of Traffic
Oe
Od R 2
Oa Ob
R 1
Of Oe R 3
Figure 31:21: Recommended Offset Dimensions for location of Traffic Islands
31.6.5 Guidelines for providing offset to the traffic islands from the
road edge
The orientation of islands near intersections is dictated by the alignment of the intersecting roadways and their associated travel paths. Proper island design must minimize the potential for vehicle impacts and reduce their severity. This is most often accomplished by offsetting the approach ends of islands from the edge of travel lane them, tapering them inward. Another technique that is the use of rounded approach noses that may also be sloped downward on their approach ends. The general design dimensions of corner islands for roadways in shown in Fig. 31:21. Another design consideration for islands is their surface finishing. Islands may be paved or landscaped. Though paved islands are easier to maintain, yet they are typically not as aesthetically pleasing. The use of colors that have contrast with the pavement surface is desirable because they allow the island to be more clearly seen by drivers. Normally concrete islands are paired with asphalt roadways and vice versa. Brick pavers are also used in areas where aesthetics are important. Other concerns include the need to provide adequate slope to the surface of the island to facilitate drainage and to keep the island free of sight obstructions and collision. Thus, all
Single Radius
Figure 31:22: Various types of curves used for a turning roadway , (a)Simple Radius (Source: [6])
landscaping features should be kept below the clear vision envelop and should not incorporate other fixed hazards.
Curve/taper combinations for turning roadways and islands
The combination of a simple radius flanked by tapers can often fit the pavement edge more closely to the design motor vehicle than a simple radius (with no tapers). Figs. 31:22, 31: and 31:24 shows the various types of curves that can be used for a roadway. The closer fit can be important for large design motor vehicles where effective pavement width is small (due either to narrow pavement or need to avoid any encroachment), or where turning speeds greater than the design speed are desired. Table. 31:3 and Table. 31:4 summarizes design elements for curve/taper combinations that permit various design motor vehicles to turn, without any encroachment, from a single approach lane into a single departure lane (Note: W should be determined using the turning path of the design vehicle) The width of the roadway can be found out from Table. 31:5 given below.
Table 31:3: Curve and Taper Corner Design Elements (Source: [9]) Angle of Turn Design Vehicle Radius Offset Taper Length (Degrees) (metres) (OS metres) (T1 metres) Passenger Car 7.5 0.6 6 75 Single Unit Truck 13.5 0.6 6 Sigle Trailor Unit 19.5 0.9 13. Passenger Car 6 0.75 7. 90 Single Unit Truck 12 0.6 6 Sigle Trailor Unit 18 1.2 18 Passenger Car 6 0.6 - 120 Single Unit Truck 9 0.9 - Sigle Trailor Unit 13.5 1.2 18
Table 31:4: Design elements for Turning Roadways (Source: [9]) Angle of Turn Design Vehicle Radius(metre) Offset (Degrees) R1-R2-R1 (OS metre) Passenger Car (P) 30-22.5-30 0. 75 Single Unit Truck (SU) 36-13.5-36 0. Semi-Trailor Unit (WB-50) 45-15-45 2 Passenger Car (P) 30-6-30 0. 90 Single Unit Truck (SU) 36-12-36 0. Semi-Trailor Unit (WB-50) 54-18-54 2 Passenger Car (P) 30-6-30 0. 120 Single Unit Truck (SU) 30-9-30 0. Semi-Trailor Unit (WB-50) 54-12-54 2.
Table 31:5: Width of roadway required for negotiating the turn for different classes of vehicles (W) (Source: [9])
Radius on One-Lane One Way One-Lane One Way Two way operation inner edge Operation (No Operation (Having Either One way or Two of provision of passing a provision of passing a way (Same Type of vehicle pavement stalled vehicle) in metre stalled vehicle) in metre in both lanes) in metre in metre P SU WB-50 P SU WB-50 P SU WB- 15 3.9 5.4 7.8 6 8.7 13.2 7.8 10.5 15 22.5 3.9 5.1 6.6 5.7 8.1 10.8 7.5 9.9 12. 30 3.9 4.8 6.3 5.7 7.5 10.2 7.5 9.3 12 45 3.6 4.8 5.7 5.4 7.2 8.7 7.2 9 10. 60 3.6 4.8 5.1 5.4 6.9 8.1 7.2 8.7 9. 90 3.6 4.5 5.1 5.4 6.6 7.5 7.2 8.4 9.
31.7 Guidelines for design of Median islands
The general guidelines to be followed in the design of median islands (separators of opposing traffic flows) are:
- The approach noses should be offset 0.6 to 1.8 m from through lanes to minimize accidental impacts.
- Shape should be based on design turning paths and island function. (Generally parabolic or circular arcs are used)
- The length of median before the intersection is related to approach speed (normally 3 sec driving time to intersection). It is also affected by available widths, taper designs and local constraints.
- The width of the medians should serve its primary intended function.
- The median should always be provided well past crest vertical curves.
Fig. 31:25 shows the general design elements of medians provided just at the approach to a intersection. The required median widths for performing their intended functions are provided by AASHTO and are shown in Table. 31:6 below. These widths are empirical and can be applied at an intersection with reasonable efficiency.
