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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.
Classification of Islands
Traffic islands usually serve more than one function, but may be generally classified in three separate types:
1. Channelizing Islands - These are designed to control and direct traffic movement, usually turning. Channelizing islands are are shown in Fig. 31:18.
2. Divisional Islands - These are designed to divide opposing or same direction traffic streams, usually through movements. Fig. 31:19 shows the placing of divisional islands in a roadway.
3. Refuge islands - Pedestrian islands are provided to serve as safety zones for the aid and protection of persons on foot. If a divisional island is located in an urban area where pedestrians are present, portions of each island can be considered a refuge island. Refuge islands are shown below I Fig. 31:20. The design aspects of the traffic islands are dealt in detail in the following sections
Design Considerations for Traffic Islands
The necessity for an island should be determined only by careful study, since it is placed in an area that would otherwise be available for vehicular traffic. The island design should be carefully planned so that the shape of the island will conform to natural vehicular paths and so that a raised island will not constitute a hazard in the roadway. A judiciously placed island at an intersection on a wide street may eliminate the need for traffic signal control by channelizing traffic into orderly movements. The total design of traffic islands can be studied in three steps:
1. 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.
2. 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 channelized the traffic and not pose any type of hazard.
3. 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.
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:
1. Traffic characteristics at the intersection
2. Cost considerations, and
3. Maintenance needs The raised islands and flush Channelization are dealt with in details in the following sections.
Flush Channelization is usually appropriate in the following conditions:
1. On high speed rural highways to separate turning lanes.
2. In constrained locations, i.e. the locations where vehicle path definition is desired but space for raised islands not available.
3. For separating opposing traffic streams of low speed streets.
4. In areas where frequent removal of snowfall is required, i.e. in places of high snow fall.
5. 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 :
1. It is not effective in prohibiting or preventing traffic movements.
2. It is also not appropriate for islands intended to serve as pedestrian refuge.
The locations where the construction of raised islands assumes importance are:
1. The primary function of the channelizing device is shielding pedestrians or to provide refuge to pedestrians crossing a street.
2. Also, the primary/secondary function is locating traffic signals or other fixed objects.
3. Intention is to prohibit or prevent certain traffic movements.
4. To separate high volume opposing traffic flows.
5. 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 Channelization is presented in Table. 31:1.
Guidelines for design of Traffic Islands
The main design principles followed for the design of the shape and size and shape of the traffic island are as follows:
1. Shape and size: Islands are generally either narrow and elongated or triangular in shape, are normally situated in areas of the roadway outside the planned vehicle paths, and are shaped and dimensioned as component parts of the street or intersection layout. The actual size differs as governed by site conditions, but the following minimum size requirements should be met to insure that the island will be large enough to command attention.
2. Traffic lanes or turning roadways should appear natural and convenient to their intended users.
3. Number of islands should be held to a practical minimum to avoid confusion.
4. The islands should be large enough to be effective. Small islands do not serve as channelizing devices and pose maintenance problems.
Figure 31:21: Recommended Offset Dimensions for location of Traffic Islands
5. These should not be introduced at locations with restricted sight distance or middle of sharp horizontal curves due to sight distance considerations. Table. 31:2 gives the recommended minimum and desired area values of the traffic islands in typical urban and rural intersections.
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
Figure 31:22: Various types of curves used for a turning roadway , (a)Simple Radius
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 paver 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 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, 31:23 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
Figure 31:23: Various types of curves used for a turning roadway, (b)Radius and Taper
Figure 31:24: Various types of curves used for a turning roadway, (c)Turning Roadway
determined using the turning path of the design vehicle) The width of the roadway can be found out from Table. 31:5 given below.
Guidelines for design of Median islands
The general guidelines to be followed in the design of median islands (separators of opposing traffic flows) are:
1. The approach noses should be offset 0.6 to 1.8 m from through lanes to minimize accidental impacts.
2. Shape should be based on design turning paths and island function. (Generally parabolic or circular arcs are used)
3. 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. 4. The width of the medians should serve its primary intended function.
5. 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
Table 31:5: Width of roadway required for negotiating the turn for different classes of vehicles (W)
Figure 31:25: Design Criteria for raised median approaches to intersections
by AASHTO and are shown in Table. 31:6 below. These widths are empirical and can be applied at an intersection with reasonable efficiency.
Auxiliary lanes are used under conditions of relatively high traffic volumes in the intersections. In these cases, traffic congestion problems can be significantly alleviated with auxiliary lanes to handle turning movements. The median lane should be 12 feet (3.6m), but not less than 10 feet (3.0m) wide and should be clearly marked for this purpose.
Auxiliary lanes can also be introduced to provide for both left turns and right turns at intersections. The need for such lanes is determined by capacity analysis and the acceptable level of service designated for the facility. The lanes should be at least 2.7m wide for reconstruction and resurfacing projects and at least 3.0m, preferably 3.6m for new construction projects. Auxiliary
Figure 31:26: Components of Auxiliary Lane
lane shoulders can be reduced to 0.6 m wide on rural sections and 0 m wide on sections with curb and gutter. The length of auxiliary lanes consists of five components:
1. Approach Taper
2. Deceleration Length
3. Bay Taper
4. Storage Length, and
5. Departure Taper.
A typical auxiliary lane with the components are shown in Fig. 31:26 below. These are discussed in detail in the following section.
1. Approach Taper- The length of the approach taper varies with operating speeds. Guidelines for determining lengths are: (i) For speeds 70 kmph and over: L = 0.6WS, and (ii) For speeds under 70 kmph: L = WS2/100 where, L is the length of entering taper in m, W is the width to be tapered in m, and S is the operating Speed in kmph.
2. Deceleration Length- The deceleration length is that required for a comfortable stop of a vehicle from a speed that is typical of the average running speed on the facility. The Bay Taper can be considered part of the deceleration length. AASHTO has again given a table for calculating the decelerating length value from the design speed value (Table. 31:7).
3. Bay Taper - This is a straight line taper with ratios varying from 5:1 to 10:1. Higher speed facilities should generally have longer tapers. Empirically, the minimum and maximum values of bay taper are taken as 18m and 36m respectively.
4. Storage Length - The storage length should be sufficiently long to store the number of vehicles likely to accumulate during the average daily peak period. (a) At unsignalized intersections, length to be based on the number of vehicles likely to arrive in an average 2-minute period within the peak hour. (b) At signalized intersections, the required length depends on the signal cycle length, the signal phasing arrangement and the rate of arrivals and departures of left turning vehicles.
5. Departure Taper - The departure taper is normally taken equal in length to that of the approach taper and should begin opposite the beginning of the Bay Taper.
Shape of Median Ends
Generally, two types of end shapes are used in practice:-semicircular shapes and bullet nose. The shape adopted normally depends on the effective median width at the end of the median. The dimensions of the various parameters for semi-circular and bullet nose ends area as: Semicircular- L = 2 × ControlR, R1 = M/2. Bullet-nose- L = ControlR, R1 = M/2, R2 = M/5 The criteria for the selection of median end is as given below in Table. 31:8. The two shapes
are illustrated in Figs. 31:27 and 31:28. The designer should evaluate each intersection to determine the best median opening shape that will accommodate the design vehicle.
Design of Median Openings
Median openings, sometimes called crossovers, provide for vehicular crossings of the median at designated locations. The design of a median opening should be based on traffic volumes and types of turning vehicles. Cross and turning traffic must operate in conjunction with the through traffic on the divided highway. This requirement makes it necessary to know the volume and composition of all movements occurring simultaneously during the design hours. The design of a median opening becomes a matter of considering what traffic is to be accommodated, choosing the design vehicle to use for layout controls for each cross and turning movement, investigating whether larger vehicles can turn without undue encroachment on adjacent lanes and, finally, checking the intersection for capacity. If the capacity is exceeded by the traffic load, the design must be expanded, possibly by widening or otherwise adjusting widths for certain movements. Traffic control devices such as yield signs, stop signs or traffic signals may be required to regulate the various movements effectively and to improve the efficiency of operations. Median openings at close intervals on other types of highways create interference with fast through traffic. Median openings should be spaced at intervals no closer than 500 m. However, if a median opening falls within 100 m of an access opening, it should be placed opposite the access opening. Also, the length of median opening varies with width of median and angle of intersecting roads. Fig. 31:29 shows the intersection median opening. The median openings for the different classes of design vehicle are as given in the Table. 31:9.
Table 31:9: Median Openings
Developing a Channelization Plan
1. Channelization is more of an art rather than science. Every intersection requires a special study because of variations in physical dimensions, turning movements, traffic and pedestrian volumes, type of traffic control etc.
2. In the next step several island configurations are considered and compared. Then a choice is made between curbed, raised islands and flush Channelization or pavement markings.
3. Next it must be checked that the design is compatible to handle turning movements of large vehicles. Also, it should be such that the vehicles are guided in normal wheel paths, so that the island does not create an obstruction in the roadway.
4. Signing and marking are redesigned to guide drivers and avoid confusion.
5. The final plan includes details of civil and electrical engineering features (like drainage facilities, curbs, lighting, signals etc.) required for the project completion.
Typical Channelization Examples
Some typical Channelization ways used in practice are as given below. Figs. 31:30 to 31:41 indicate both normal Channelization and high type Channelization techniques for various intersections and situations.