When two or more roads intersect or cross, the users of these roads have to necessarily adjust their movement in order to avoid ‘collision’; this adjustment could be by way of reduction in speed and/or change in the path of their motion.
An intersection is defined as “the general area where two or more highways join or cross, within which are included the carriageway and roadside design features which facilitate orderly traffic movements in that area”. An intersection leg is “that part of any one of the highways radiating from an intersection which is outside of the area of the intersection proper.”
Depending upon the direction or intersection leg towards which the driver of an approaching vehicle wants to travel, he or she has to make certain kinds of traffic manoeuvres at the intersection area. These have been referred to in Section 4.3 under Traffic Flow Characteristics.
Thus, depending on the manoeuvres chosen by individual drivers, conflicts tend to occur between their vehicles; these have been referred to in a simple manner in Section 4.4 on Traffic Regulation and Control Devices. Traffic conflicts tend to occur at or near intersections; they are potential sites of accidents and are therefore important in the consideration of road safety. Figure 5.1 shows traffic conflict points at intersections.
The area in which the conflict points fall is called the conflict area. The outer part of this where only two vehicles conflict is the minor conflict area and the inner part where more than two vehicles tend to conflict is the major one.
Types of Highway Intersections:
Highway intersections may be broadly classified into the following two types:
1. Intersections-at-Grade:
Also called ‘at-grade intersections’, all the areas/legs of the intersection join or cross at the same level. The basic traffic manoeuvres such as diverging, merging and crossing are used in at-grade intersections.
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The basic forms of at-grade intersections are shown in Fig. 5.2.
Sometimes, the alignment may be modified to achieve right-angled (or near about) crossings.
These are very common in view of their relative economy vis-a-vis grade separations. The relevant IRC recommendations are contained in “IRC-SP 41:-Guidelines for the Design of At-grade Intersections in Rural and Urban areas, Indian Roads Congress, New Delhi, 1994”.
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Visibility at Intersections:
This aspect has already been covered in Section 3.4 under ‘Lateral or Intersection Sight Distance’. When both the intersecting roads are of equal importance, the sight distance for the driver approaching the intersection is the safe SSD for the design speed on the particular arm. The diagonal of the sight or visibility triangle should be clear of any obstruction for non- signalised intersections.
In the case of a priority junction, the traffic on the minor road is controlled by STOP or GIVE WAY traffic sign and/or road markings. The visibility distance for drivers approaching from the minor road should be adequate for their reaction and judgment of the traffic on the major road for a safe crossing. The current Indian practice is to maintain a minimum visibility distance of 15 m along the minor road.
The corresponding distance along the major road is considered to be a time-headway of 8 seconds; or, the minimum visibility distance along the major road is the distance travelled by the vehicle at its design speed in 8 seconds. Thus, for a speed of 100 km/h, it is 270 m; for 65 km/h, it is 145 m; and for 50 km/h, it is 110 m. In urban sections, these speeds are further reduced.
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Width of Carriageway at Intersections:
It has already been seen that the width of the carriageway needs to be increased on horizontal curves to counteract the ill-effects of the centrifugal force (Section 3.5). Similarly, near intersections at-grade, the turning moments are facilitated by introducing sharp curves to be negotiated at a minimum speed. Here also, an increased carriageway width is found necessary. Table 5.1 gives the particulars of the inner radius at the corners, design speed, and lane widths for single-lane and two-lane roads.
As the angle of turn increases, the radius of the corner curves decreases, the curves becoming sharper and sharper.
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Speed-Change Lanes:
While entering into or leaving an intersection, drivers have to necessarily change their speed. While entering into an intersection, the speed is reduced to a safe limit at which the intersection may be negotiated, while leaving, the speed has to be increased until the desired design speed on the highway is reached. If such deceleration or acceleration is accomplished on the regular carriageway, traffic could be disrupted and even hazards or accidents might occur.
To prevent this, ‘speed-change lanes’ are provided on superior highways like expressways or on national highways. Such lanes also increase the capacity of the intersection. IRC recommends that speed-change lanes be provided if the projected traffic on these lanes is more than 1000 PCU’s per day. Speed-change lanes are either acceleration lanes or deceleration lanes.
An acceleration lane enables a vehicle entering a lane to increase its speed and merge safely with through traffic. A deceleration lane is an auxiliary lane to enable a vehicle leaving the through traffic stream to reduce speed without interfering with other traffic; such a lane is provided on the near-side for left-turning traffic.
Acceleration and deceleration lanes are usually provided with a taper, as shown in Fig. 5.3. The length of a deceleration lane depends upon the manoeuvring speed and the deceleration characteristics; the length of an acceleration lane depends upon the speed at which the drivers merge with through traffic and the acceleration characteristics.
Table 5.2 gives IRC recommended lengths of deceleration and acceleration lanes.
Channelisation:
Channelisation means “directing the traffic flow at intersections to specified paths by means of traffic markings, traffic islands, or such traffic control aids.”
An intersection that is channelised to direct traffic into definite paths by markings and islands is referred to as a channelised intersection. Evidently, an intersection that is not treated in this manner and left in its bare form is said to be unchannelised.
Objectives of Channelisation:
The objectives of channelisation of intersections are:
(i) Reducing the number of possible vehicle conflicts and areas of conflicts in the carriageway, and presenting the drivers with only one option at a time.
(ii) Controlling the angles of crossing such that acute crossings, which are hazardous, are avoided.
(iii) Reducing the approach speeds of vehicles at the intersection and increasing the exit speeds from the intersection.
(iv) Providing separate storage pockets for right turning traffic at the time of leaving or crossing the main traffic flow.
(v) Providing a channelising island to serve as a refuge and as a protection for pedestrians crossing the intersection.
(vi) Reduction of large paved areas in order to curb the driver’s tendency to make hazardous movements.
(vii) Making prohibited movements impossible or at least inconvenient.
(viii) Providing space for erection of traffic control devices such as direction signs and other informatory signs.
Figures 5.4-5.6 illustrate some of the measures of channelisation adopted to fulfill a few of these objectives.
Salient Features of Channelising Islands:
Channelising islands (Figure 5.7) can be of different shapes and signs to suit the geometry of the intersection. The minimum area for a channelising island should be 5 to 7 sq. m. The minimum width is 1.2 m and the length can be 3.5 to 6 m.
Refuge islands, meant for protection of pedestrians, should be offset 0.6 m from the edge of the carriageway.
The approach nose of the islands should be rounded off to a minimum radius of 0.6 to 0.9 m; the merging end nose should be rounded off to at least 0.3 m radius. Diagonal markings and chevrons are used to indicate the nose to drivers for additional guidance. (Fig. 5.8 and 5.9).
Cross-Roads:
Cross-roads with islands and driving channels is a common treatment (Fig. 5.10).
T-Junction:
A plain unchannelised T-junction is suitable only for minor roads with very light traffic. For high-speed traffic and large turning movements, this is very hazardous and prone to accidents and collisions.
Common measures adopted to improve performance at the intersection include flaring of the main road, introduction of islands and turning roadways, divisional islands, and channelisation. Some of these are illustrated in the figures that follow (Figs. 5.11 to 5.13).
Y-Junction:
The treatment of a Y-junction is similar to that of a T-junction except that here, the intersection angle is very acute.
Figs. 5.14 and 5.15 are two possible treatments for Y-junctions.
2. Grade-Separated Intersections:
Also known as grade-separated intersections, the roads are separated and constructed at different elevations, obviating the need for crossing at the same elevation. Grade separation may be achieved by the construction of an over-bridge or an under-bridge; when one or more of the crossing highways are taken at an elevation higher than the general ground level at which the others are constructed, an over-bridge is needed for the ones at the higher level; when they are taken at an elevation lower than the general ground level, an under-bridge fulfills the purpose. All crossing conflicts are thus automatically eliminated.
Transfer from one road to another is facilitated by interchanges consisting of ‘ramps’. Bridge structures for grade separations may be created using T-beams, arches, rigid and portal frames, or pre-stressed concrete. The vertical clearance should be at least 4.3 m to 5.2 m. Site conditions and aesthetic considerations must also be considered in selecting the bridge structure to maintain the necessary difference in level.
Grade separations are more expensive initially; but they are justified in the following circumstances:
i. The existing at-grade intersection has reached its maximum capacity, which cannot be improved further.
ii. The particular location has a very bad record of accident history as an at-grade intersection.
iii. There is considerable economic justification for a grade separation in view of very heavy traffic volume and the loss caused by delays.
iv. The topography of the location involves considerable earthwork or land acquisition for an at-grade intersection.
v. The facility is a high-end type such as an expressway or a freeway with through fast traffic.
The site conditions justifying the provision of grade-separated intersections have already been given in Section 5.2. IRC gives guidelines for providing grade separations in urban streets and rural highways.
In urban streets, grade separation is to be provided if the estimated traffic within the next five years exceed the present capacity of the intersection. If the traffic projections show that, in the next 20 years, traffic volumes exceed the maximum capacity of an at-grade intersection, a grade separation is indicated. In rural highways, a grade separation should be provided at intersections of divided rural highway if the average daily traffic (ADT) on the cross road is expected to exceed 5000 PCU within the next 5 years.
Grade-separation is of great importance across railway tracks. IRC: 62 – 1976 recommends grade-separation across existing railway lines if the product of ADT and the number of trains per day exceeds 50,000 in the next 5 years.
For new constructions such as bypasses, grade separations shall be provided when this figure exceeds 25,000.
Types of Grade-Separated Intersections:
There are two types of grade-separated intersections:
(a) Grade-separated intersections without interchange.
(b) Grade-separated intersections with interchange.
Interchange is a facility for movement of traffic between two or more roads at different levels in a grade-separated junction.
(a) Grade-Separated Junction without Interchange:
This is a system wherein the traffic at different levels moves separately without a provision for an interchange between them; the separation is achieved by means of an over-bridge, fly-over, or an underpass.
(b) Grade-Separated Junction with Interchange:
This is a system wherein the traffic, besides moving separately in streams at different levels, can get interchanged from one stream to another via an interchange facility. A proper design of the system facilitates an orderly and safe movement of traffic. This is a high-end facility for large volumes of traffic on two or more roads involved with a heavy proportion of turning traffic.
Interchanges can be considered on the basis of the number of legs served by the intersection and classified as three-leg, four-leg, and multi-leg intersections with the following subdivisions:
(1) Three-Leg Interchange:
(i) T-type
(ii) Y-type
(iii) Rotary type
(2) Four-Leg Interchange:
(i) Diamond type
(ii) Partial clover-leaf
(iii) Clover-leaf
(iv) Rotary
(v) Directional interchange
(3) Multi-Leg Interchange:
Rotary interchange
(1) Three-Leg Interchange:
A T-interchange is one in which one of the intersection legs meets a highway approximately at right angles, but does not cross it, and it is provided with an interchange facility.
A Y-interchange is similar to a T-interchange, but for the intersection angle which is acute or obtuse (Fig. 5.21).
A widely used form of T- or Y- interchange is a ‘Trumpet’ (Fig. 5.20).
(2) Four-Leg Interchange:
This is an interchange with four intersecting legs. The simplest of these is the diamond interchange (Fig. 5.22).
When the crossing involves major and minor roads in an urban area, a diamond interchange is popular.
Half Clover-Leaf Interchange:
Also called a partial clover-leaf interchange, this type is suitable when a major road crosses a minor one with not more than two lanes (Fig. 5.23).
Clover-Leaf Interchange:
The clover-leaf interchange is four-leg interchange which fulfills all the requirements of turning traffic with simple traffic manoeuvres. This is perhaps the best solution when two highways with heavy traffic volume and high speed intersect each other (Fig. 5.24).
Merits:
1. Only one structure is necessary.
2. Left-turning traffic has a direct path.
3. Through traffic on both the roads has unobstructed way
4. Easy to follow the operations without confusion.
Demerits:
1. A very large area is required for the layout.
2. Weaving manoeuvres are involved in both the roads.
3. Long U-turns cause inconvenience in operation.
4. Right-turning traffic has to travel larger distances.
5. Weaving capacity is limited to about 1200 PCU/hour.
6. Loop design speeds and loop capacities are restricted.
Grade-Separated Rotary Interchange:
This can serve as a four-leg interchange or as a multi-leg interchange; a typical form of the latter is shown in Fig. 5.25.
Merits:
1. U-turns are easy.
2. Carriageway area is less than that in many other interchanges.
3. Occupies relatively less area.
Demerits:
1. The capacity of the interchange is limited by the capacity of the round-about.
2. Straight traffic on road is required to weave through turning traffic from the other.
3. The larger the number of legs, the larger will be the confusion in operation.
Directional Interchange:
A directional interchange facilitates direct or semi-direct connections for major right-turning movements; but these interchanges are rather complex and require multi-level support structures.
A schematic of a directional interchange is shown in Fig. 5.26.
Design Criteria for Grade Separations:
For the purpose of design, the grade-separated interchange consists of three major elements:
(i) Exit terminal – The point of leaving through road
(ii) Ramp – It may be a single-way road leading to the through road; but, in the roundabout type, this includes the roundabout itself.
(iii) Entry terminal – The point of joining the through road.
When two levels are involved, a decision has to be made regarding which road is to be taken over the structure and which one under the structure. The criteria which help in this regard are the topography, major traffic movements, type of the highways, and economic considerations. An overpass gives less feeling of restriction and has practically no problems of drainage. On the other hand, an underpass is better when the major road is at about the general ground level, but drainage problems have to be given due attention.
Design Criteria for Ramps/Interchanges:
The following are the common terms used in this connection:
(i) Interchange ramp – An interconnecting roadway or any connection between roads at different levels. It may be in the form of a loop, outer connection or direct connection.
(ii) Loop – A one-way turning road that curves about 270° to the left to facilitate a right-turning movement.
(iii) Outer connection – A ramp used by the left-turning traffic to move from one through roadway to another through roadway, separated by a grade separation structure.
(iv) Direct connection – A ramp which does not deviate much from the intended direction of movement. It avoids the loop for right-turning movement. An outer connection acts as a direct connection for left-turning movement.
These are shown schematically in Fig. 5.27.
Design Speed for Ramp:
For ramps, the design speed will be less than that for the highway. For example, for a design speed of 80 km/h for the highway, that for the ramp will be 40-70 km/h. For loops, it is only 45 km/h; for direct connections, a design speed of 65 km/h is considered appropriate.
Sight distance – A minimum stopping sight distance appropriate for the design speed is necessary.
Gradient on ramps – It should be limited to 6% in areas subject to snow; it should not exceed 4% for heavy truck traffic. In the case of minor ramps with low traffic volumes, an exceptional gradient of 10% may be provided.
Width of ramp – Since a ramp is expected to carry one-way traffic only, the width should be for the lane along with an extra-width for the curvature, if any.
Capacity of ramp – The practical capacity of through lanes may be taken as 1500 PCU/hour per lane for interchanges. A value of 1200 PCU/hour is considered appropriate.
The capacity of weaving sections may be got from the principles given in the Highway Capacity Manual, 1965.