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 capacity, 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.
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:
Consider for example the T-intersection shown in Figs. 2, 3, and 4. In Fig. 2, the intersection has no special Channelization for helping drivers in avoiding conflicts between movements. In Fig. 3, a passing lane for through vehicles in the eastbound direction and a westbound right-turn lane has been added, which helps in separating the turning traffic from the through ones. In Fig. 4, the use of lanes is further clarified due to the addition of channelizing islands.
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:
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.
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.
Traffic islands usually serve more than one function, but may be generally classified in three separate types:
The design aspects of the traffic islands are dealt in detail in the following sections.
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:
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:
The raised islands and flush Channelization are dealt with in details in the following sections.
Flush Channelization is usually appropriate in the following conditions:
However, the main demerits of this type of Channelization are :
The locations where the construction of raised islands assumes importance are:
A comparison between the usefulness and the operating conditions of the two types of Channelization is presented in Table. 1.
FLUSH CHANNELIZATION | OPERATING CONDITIONS |
1. For Right turns | High Speeds |
2. To provide temporary or trail Channelization | Rural highway |
3. To shadow left turns | Minor urban intersections |
RAISED CHANNELIZATION | OPERATING CONDITIONS |
1. Post signs or signals | Urban streets |
2. Provide pedestrian refuse | Low speeds |
3. Prevent wrong way movements | High volumes |
The main design principles followed for the design of the shape and size and shape of the traffic island are as follows:
Table. 2 gives the recommended minimum and desired area values of the traffic islands in typical urban and rural intersections.
Location of Intersection | Size(Sq.meters) | |
Minimum | Desired | |
Urban | 4.65 | 7 |
Rural and High Speed urban/Suburban | 7 | 9.3 |
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. 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 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.
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. 22, 23 and 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. 3 and Table. 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)
Angle of Turn | Design Vehicle | Radius | Offset | Taper Length |
(Degrees) | (meters) | (OS meters) | (T1 meters) | |
Passenger Car | 7.5 | 0.6 | 6 | |
75 | Single Unit Truck | 13.5 | 0.6 | 6 |
Single Trailer Unit | 19.5 | 0.9 | 13.5 | |
Passenger Car | 6 | 0.75 | 7.5 | |
90 | Single Unit Truck | 12 | 0.6 | 6 |
Single Trailer Unit | 18 | 1.2 | 18 | |
Passenger Car | 6 | 0.6 | - | |
120 | Single Unit Truck | 9 | 0.9 | - |
Single Trailer Unit | 13.5 | 1.2 | 18 | |
Angle of Turn | Design Vehicle | Radius(meter) | Offset |
(Degrees) | R1-R2-R1 | (OS meter) | |
Passenger Car (P) | 30-22.5-30 | 0.6 | |
75 | Single Unit Truck (SU) | 36-13.5-36 | 0.6 |
Semi-Trailer Unit (WB-50) | 45-15-45 | 2 | |
Passenger Car (P) | 30-6-30 | 0.8 | |
90 | Single Unit Truck (SU) | 36-12-36 | 0.6 |
Semi-Trailer Unit (WB-50) | 54-18-54 | 2 | |
Passenger Car (P) | 30-6-30 | 0.6 | |
120 | Single Unit Truck (SU) | 30-9-30 | 0.9 |
Semi-Trailer Unit (WB-50) | 54-12-54 | 2.6 | |
The width of the roadway can be found out from Table. 5 given below.
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 meter | stalled vehicle) in meter | in both lanes) in meter | ||||||
in meter | P | SU | WB-50 | P | SU | WB-50 | P | SU | WB-50 |
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 | 12.6 |
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 | 10.5 |
60 | 3.6 | 4.8 | 5.1 | 5.4 | 6.9 | 8.1 | 7.2 | 8.7 | 9.9 |
90 | 3.6 | 4.5 | 5.1 | 5.4 | 6.6 | 7.5 | 7.2 | 8.4 | 9.3 |
The general guidelines to be followed in the design of median islands (separators of opposing traffic flows) are:
Fig. 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. 6 below. These widths are empirical and can be applied at an intersection with reasonable efficiency.
Function | Width in meter | |
Minimum | Desirable | |
Separation of opposing traffic | 1.2 | 3 |
Provision of pedestrian refuse | 1.8 | 4.2 |
Provision of storage for left-turn vehicles | 4.8 | 6 |
Provision for protection of vehicles crossing | 7.5 | 9 |
through lanes | ||
Provision for U turns, inside to outside lanes | 4.8 | 6 |
Provision for U-turns, inside to inside lanes | 7.8 | 9 |
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 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:
A typical auxiliary lane with the components are shown in Fig. 26 below.
These are discussed in detail in the following section.
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: Semi-circular- 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. 8.
Effective Median Width | Median End Shape |
Less than 3m | Semi-circular |
3m - 20m | Bullet Nose |
Over 20m | Treated as a separate intersection |
The two shapes are illustrated in Figs. 27 and 28. The designer should evaluate each intersection to determine the best median opening shape that will accommodate the design vehicle.
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. 29 shows the intersection median opening.
The median openings for the different classes of design vehicle are as given in the Table. 9.
Width of | Passenger Car | Single Unit Truck | Single Trailer Unit | |||
Median(m) | Semi - circular | Bullet nose | Semi - circular | Bullet nose | Semi - circular | Bullet nose |
1.2 | 22.8 | 22.8 | 28.8 | 28.8 | 43.8 | 36.6 |
1.8 | 22.2 | 18 | 28.2 | 22.8 | 43.2 | 34.5 |
2.4 | 21.6 | 15.9 | 27.6 | 20.4 | 42.6 | 33 |
3 | 21 | 14.1 | 27 | 18.6 | 42 | 31.5 |
3.6 | 20.4 | 12.9 | 26.4 | 17.4 | 41.4 | 30 |
4.2 | 19.2 | 12 | 25.8 | 15.9 | 40.8 | 28.8 |
4.8 | 18 | 12 | 25.2 | 15 | 40.2 | 27.6 |
6 | 16.8 | 12 | 24 | 13.2 | 39 | 25.5 |
Some typical Channelization ways used in practice are as given below. Figs. 30 to 41 indicate both normal Channelization and high type Channelization techniques for various intersections and situations.
In the design of intersections the turning paths of vehicles assumes utmost importance. The turning paths of design vehicles are given in transparent templates such as the one shown in Fig. 17 and Fig. 18. These templates are placed over the intersection plan to trace the path of the turning vehicle. Once this is done, proper islands and other traffic control devices can be designed. As per AASHTO, the turning templates are drawn at an approximate scale of 1”=50’. The radius of the template is measured to the outside front wheel path at the beginning of the curve. The design vehicle for the purpose can be taken out of a list of 16 different types of vehicles suggested by AASHTO. The dimensions of some of the design vehicles are given in Table. 10 below. The templates are applied to the layout of intersections and other facilities in accommodating vehicle maneuvers, including driveways, car parking, truck loading and bus terminals.
Design Vehicle Type | Symbol | Overall Dimension | ||
Height (m) | Width (m) | Length (m) | ||
Passenger Car | P | 1.3 | 2.1 | 5.8 |
Single Unit Truck | SU | 4.1 | 2.6 | 9.1 |
Single Unit Bus | BUS | 4.1 | 2.6 | 12.1 |
Intermediate Semi-Trailer | WB-15 | 4.1 | 2.6 | 16.7 |
Here we shall take the cases of a passenger car (P) and a single unit truck (BUS) as the design vehicles. The various design elements and their dimensions are shown in Fig. 42 and Table. 11 respectively.
Vehicle | WB | Minimum Turn | |||||||
Designation | L(m) | (m) | A(m) | B(m) | W(m) | U(m) | U** (m) | FA | RT |
(m) | (m) | (m) | |||||||
BUS | 12.1 | 7.5 | 2 | 2.5 | 2.6 | 2.6 | 4.98 | 1.25 | 13 |
Passenger Car (P) | 5.8 | 3.4 | 0.9 | 15 | 2.1 | 1.8 | 2.61 | 0.6 | 7.5 |
The templates were developed to include a variety of angles, with specific configurations for every 30 degrees of turn (30, 60, 90, 120, 150 and 180). By special manipulation of the template, any degree of turning can be produced within an overall range of 20 to 200 degrees. The four variables-vehicle type, turning radius, angle of turn and scale-provide full flexibility in the use of turning vehicle templates for layout and design. To permit greater latitude in maneuvering of buses, single unit trucks and passenger cars, special bar tenders are included, consisting of turning radii in the range of 13 to 50 meters for the first two and 5.5 to 30 meters for the last type of vehicles which are outside the scope of this discussion. The list of templates for bus and passenger cars is shown in the Table. 12.
Vehicle Type | Scales | Turning Radius-m | Average Size-cm |
1:250 | R= 13 & 18 | 20 × 25 | |
BUS | 1:500 | R= 13 & 18 | 18 × 18 |
1:250 | R=13 to 50 | 20 × 25 | |
Bar Template | |||
1:250 | R=7.5 | 18 × 18 | |
Passenger car | 1:250 | R=7.5 to 30 | 18 × 18 |
Bar template | |||
The templates for the Passenger Car (P) and Bus are as shown in Fig. 43, 44 below.
Provide Channelization for an intersection having EW as the major road. The major and minor roads intersect at right angles. The design vehicle is WB-50 (R=25m) and design speed is 45 kmph. The intersection is unsignalized. EW road has 2 lanes in each direction and NS has 1 lane for each direction. Take lane width =3.6 m. Provide bullet nose median ends. Also provide channelizing island for free right for WS bound traffic.
Solution : The approach taper for auxiliary lane is equal to 3.6 × 45 × 45∕100 = 73 m. The deceleration Taper is taken as 40 m. Considering a 1:10 taper, the Bay Taper is found out to be 18 m. Let the storage length = 30 m (say). Now from Table. 9, it is found that for bullet nose median end, Median Opening = 30 m. The dimensions of all the components of the auxiliary lane are shown in Fig. 45.
The width required for the WB- 50 semi-trailer unit is found to be about 6.5 m. Additional 0.5 m is provided on the outer side and 0.3 m is provided on the inner side away from the edge of the island. For the turning roadway for the W-S direction, the single offset method is used. At 0.3 + 0.5 + 6.5 = 7.3 m from the island edge, a circle of radius 25 m is laid out. Then two tapers of slope 1:15 is laid out on either side of the arc to join with the straight edge on either side. Thus the Channelization is provided for the W-S approach. Similar method can be used for designing the Channelization schemes of the other directions as well. The Channelization for the W-S approach is shown in Fig. 46.
Following the principles of Channelization suggest suitable island schemes for the following intersections (considering both high relative speed and low relative speed) (Figs. 47, 48)
This chapter presents one of the simple and cost effective way of intersection control, namely the Channelization. This is normally adopted for low and medium volume roads. The chapter contains the design principles, traffic islands, and median.
I wish to thank several of my students and staff of NPTEL for their contribution in this lecture. I wish to thank specially my student Mr. Pratik Patra for his assistance in developing the lecture note, and my staff Ms. Reeba in typesetting the materials. I also appreciate your constructive feedback which may be sent to tvm@civil.iitb.ac.in
Prof. Tom V. Mathew
Department of Civil Engineering
Indian Institute of Technology Bombay, India
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Wednesday 27 September 2023 11:15:37 PM IST