Lecture 45: Intersection Layouts - 3

Dear students, let us continue our discussion on intersection layouts. In the previous interaction, we discussed about the grade separated intersections, what are the various categories in which they can be divided and then we had a discussion on service interchanges towards the end. Now in today's interaction, we are going to continue from there and we will be discussing about the system interchanges and these system interchanges are another category of grade separated intersections and then after that, we are going to take up the basic design principles related to at grade intersections. So let us start with system interchanges now. Now if you remember, in the case of service interchanges, we had a definition where we said that the free flow is not going to be maintained on each and every element of that particular intersection, whereas the difference here is that this is an interchange that maintains the free flow through all of its elements for all the major movements. So that means that is a sort of a differentiation which is coming. Now when we are looking at all major movements and having a free flow condition, no restrictions at all, that means here instead of having a crossing of a major road with a minor road or a major road with a major road which is having the intersections which are at grade, in this case, they are the major road versus major road where the high traffic volume is there and in this case, the another major road is not having the at grade intersections. So it means both of those roads are of a higher category and they are going to have the grade separated intersections wherever they are required. Now when we talk about this sort of a free flow conditions or all the major movements which are going to be there which are unrestricted in nature, then that is going to be possible by way of the provision of different categories of ramps. And so as to just inform you that what categories of ramps can be here, they have been written here as like they can be directional, semi-directional or large radius loops. Now we will look at this type of a classification a little later also and we will see that what are the various types of those ramps or loops or semi-loops, etc., and how they look like in any of that interchange. Now here you can see that some of the examples of system interchanges like there is a clover leaf. So this clover leaf means you are going to have different type of movements from the 4 lags which are coming at this particular intersection and we are trying to provide a separate ramp for different that movement. Say if you are coming from this side and you are going to the left, then this is the mechanism being provided. You can go straight on this in this form. You have to go right, then that means from here we are coming and then going in this way. So this is going to be the right turning. So likewise from each and every direction we are providing certain such mechanism so that the traffic can keep moving on inert. Now there is another case which is defined as direct where for each and everything means you are going to get a direct connectivity and there is no such even this type of a condition where you are getting a merging and then going forward in that. The mergings of course are going to be there because the ramps has to be either exit or enter at a particular location. But then the movements are totally direct on inert in this case. Like this is another case where a y type is being shown and what you found as there at this location or at this location or at this particular location, there are different levels. So it is a 2 level system, it can be a 3 level system also, it all depends how many type of directional movements or direct directional movements we are interested to provide. Let us look at this example where is a T interchange is being provided. In this T interchange if we are coming from this direction, so what you see is that the traffic has to go to the right. So a elevated system has been provided and this is taking the traffic towards the right. Say if I just make a change here, so this is the movement and then we are going with this and coming here and then after this getting merged into the system. But if the traffic has to go left, then there is another system which can be provided. So there is a ramp which is going in this form. The traffic which is coming from this direction, it can move straight like this or if it has to go towards the right hand side, then this is the one which is going to take and then get into the system here. Similarly the traffic coming from here is taking a left and it is going straight in this form. So what we may find is that at this particular level, we are having a 2 level system. Here again we have a 2 level system, here also we have a 2 level system. So depending on what type of directional connectivities are required, we have to go into these type of changes in the vertical direction. But yes, as soon as you start doing this as a multilevel system, the cost of the project, the cost of that intersection is going to increase a lot. So this is an issue which is going to be there. So it all depends how much amount of money is available in our hand and therefore we can do that in this form. Now when we talk about these system interchanges based on the number of lags or approaches which are there at a particular intersection, they can be defined as a 3 lagged or a 4 lagged interchanges. Now 3 lagged interchanges, obviously when you have 3 lags, so it can be either this way or it can be in this form. So it can be a D type of a configuration or a Y type of a configuration. But then when you are looking at the various types of connectivities, the various types of movements, it will require large loops as well as semi-directional ramps which are having large radius. So when we are looking at these type of connectivities, so that means this is the way it is going to be there or it is coming in this form. So if this is the type which is being provided, then obviously the radius is going to be quite high at this one. But then you have loops, semi-loops, etc. And this may also require catering for a frontage road depending on the type of the development which is there. Whereas in the case of 4 lagged interchanges, different designs can be there. One design which we have seen in the previous one was a clover leaf, another can be a diamond type and then the things can be converted into a directional or a semi-directional interchanges or there may be a combination of such interchanges which can be defined as composite interchanges. Now these type of interchanges when they are there, they require a lot many combinations of maneuvers and accordingly the different type of the ramps which are being defined either in terms of a direct connectivities, semi-direct connectivities or loops. So that is the issue which is going to be there with these. But as I said in all of these cases, the cost is going to be one issue because you are going with a multilevel structure. So this is one example which we have of course seen previously also. So if we are looking at this one and the traffic is coming from this direction say, so this is the traffic say if coming and it has to go on the right hand side, then this is going to take the path in this form or a traffic which is coming from say this direction it has to go in this direction. So very easily it can happen in this way. For the traffic which is coming from this one has to go right, then there is a structure from here and then it is going in this direction. So what you found at this particular level, now it is getting becoming a multilevel system. So you have a ground level, then you have a elevated road here and then another elevated road here. So you have different levels at this one, there also there are different levels, at this point there are different levels. So but the beauty of this is that it is going to provide you a fast connectivity but yes the space which is required to do all of these juggleries is quite large. This is a cloverleaf arrangement where this main highway is moving like this at the top and the bidirectional traffic can be there. Now if we are going with our way of movements on the roads that means we have the type of driving opposite to what is being done in many of the other countries. Then if you are coming from this direction, then you can have a left movement in this form. A straight movement is already being defined. Now if we have to go to the right, then what is to be done? We have to get out of the system here, take this loop and get here and then move in this one. So at this particular location, we have an area where we are either accelerating or if we are coming from this direction and going out of this one, then here we are first of all deaccelerating getting this loop and then going in the other direction. So all of these loops from all of the 4 directions, they are providing you these type of connectivities and that is going to be one of the easiest and the fastest way of providing services here to different type of users. This is a rotary design and in this rotary design what you can see is the traffic is merging or diverging tangentially to this rotary which is at an elevated location. So you are getting into an embankment. So this is an embankment area. So the traffic is going in and out through these tangential systems and they can move in this form or they can move in this way. So if they are moving in this form and there is a traffic coming out of it that means at this particular point there is going to be weaving and here it is going to be diverging and at this point it is going to be merging. So different type of movements are going to be there when you are designing this type of a system. This is a partial system. So what we found is only 2 clover arrangements have been provided, not many all directions have been provided. This may be because there is no requirement of doing that and the traffic which has to go to the say the particular direction. See if the traffic is coming from here and it is taking this loop and going this way so that right directional traffic. So from this one it is probably required. Similarly when it is coming from this direction and then going and then in this form it is moving. So only these particular 2 directions is requiring this type of a movement but if any vehicle which is coming from the other direction like this has to go the other side then this has to come at this level where at grade arrangement has to be done and at this at grade arrangement this vehicle may be provided a flexibility to take a turn and through this ramp then get into towards this direction. So it all depends that what are the major movements which needs to be controlled and accordingly what type of facilities to be provided. So this is another typical design which is a sort of a combination of add grade and grade separated as well as crisscross movements being shown. What you can see is that the traffic is moving in this form and then this form. So there is type of a crisscross conditions are there at this level there is an elevated condition whereas here the junction which is provided that is coming at same level from all different directions. So again this is one another type of a design which can be provided. So it is a cross as well as other thing. Now let us come to the design principles that is the second part which we thought of taking up. So these design principles which we are going to look here they are related to add grade intersections actually. So this is one example of an add grade intersection a wide intersection area has been shown and multi lane system is there you can see that there are 3 lanes through which the traffic is moving in a particular direction there somewhere around like again 3, 4 lanes for the traffic moving in these directions and then a median is being provided and that median has a curve there is an island at this side there is an island at the other side there is a median with a particular design. So what is going to happen is that when you are trying to look at all of these things a lot many things has to be designed and implemented in this particular intersection area apart from the pavement markings which we discussed a way back. So let us look at that what are the things which you need to take care of. Now when we talk about these design principles then the things we are going to talk about is the uniformity and the simplicity of the design the another thing is because there are lot many movements which are happening in terms of merging, diverging, weaving etc. or crossing movements are also there. So we have to reduce the conflicts. So the number of conflicts and the area of conflict needs to be reduced. Then another thing is that the safety of each and everything that means the road users as well as the property or the furniture which is being provided as a part of our road element needs to be taken care of. And then we have to see that whatever the roads which are intersecting each other what is the alignment of those roads. Is it conducive for doing the maneuvers in a smoother way or not? Are those maneuvers going to be safe? If there is an issue then we have to go for a realignment. So an alignment in profiles is another issue which needs to be talked of and then the various types of controls which are exercised this is also a part of the design principles. So in this area let us first of all discuss few more things related to uniformity and simplicity. Now in the case of uniformity and simplicity as soon as we say that it is simple whatever design we have provided is simple means what that it is not creating any confusion in the minds of the drivers, in the minds of the different road users at that particular location. Now when we are talking about this design then this design can be such that it defines that what is the speed which needs to be taken up means they have to move at 30 kilometers per hour or 40 kilometers per hour or so on which particular path is to be taken for different type of movements. There are turning movements, left turning, right turning or there are crossing movements. So how the paths have been defined so that the safety can be ensured and so as to ensure these things that is the control on the speed as well as control on the path what type of control devices needs to be placed at that level and then what is the geometric feature which are provided, what are the features which are visible to a person as a driver as a road user and the top of the carriage way that means those physical features also should be such that they define certain things to the driver in a most simpler manner. Now when we say that we are having different type of such elements at a particular location and here our location means intersection then in that case whatever the things have been done they should be universal in nature. So we have discussed about this component previously also when we talked about signs when we talked about markings and when we started our interaction on the geometric design from the very first lecture itself then we did talk about self-explaining roads and we said that these self-explaining roads are going to be there on the basis of the various components, various elements which have been implemented on the roadway and they are defining how to move, in which direction to move, when to move, where not to move and all those things. So these things needs to be universal in nature, it should not be site specific. If something has happened at a particular location then if it is expected to happen at some other location the same sort of design should be visible and therefore depending on what type of flow movements are going to be there what we are trying to say is that it should culminate into the similar type designs and if culminates into the same then on the basis of the experience with respect to those designs the user or the driver is going to be more comfortable in traversing and negotiating those designs rather than having something else and which causes to make them understand first of all and then make the maneuver. Now when we are trying to provide the things as a traffic planner or traffic engineer then we should not try to impose the things on the users rather we should try to see the capabilities of different road users. Say we are talking about a crossing facility for a pedestrian and we assume that the walking speed is something like 1.2 meters per second and we are now making an assumption that I am going to provide this crossing with a signal and their speed should not be less than 1.5 meter per second is not the way we should try to do it or there is a natural movement of a driver under a certain condition and we restrict that and ask the driver to do something else then that is not the way we should try. So what we need to look at is that what are the capabilities as the same time the different users have different type of limitations. So the steering limitation because of the size of the vehicle we have to take into consideration we cannot ask a truck to just make a maneuver in terms of a left or a right turning where the space is being provided in a more restricted manner which is only conducive to motorcycles or maybe at the maximum for cars. So the limitations and capabilities of the users needs to be taken into consideration and based on that the design needs to be done. If that is the way the designing is being done then this is going to be more acceptable in nature. So the acceptability will improve. Now another thing is it should not be based on expectation to follow that is where I said that it should not be on the basis of the planners thought process which is imposing in nature. So that should not happen. Not complex requiring decision making, decision making means you have to first of all stop your vehicle try to understand what is happening in front of you and then you start moving by that time there is a queue being formed at the back of your vehicle. So that is what is there. It should not make it so complex. Yes there are requirements of different type of movement, different type of maneuvers we are going to have say interchanges etc. where for each and every movement there is a different level. But if it is being provided in proper manner with complete information system then the decision making is going to become more easier. So when we are looking at this uniformity and simplicity that what are the various elements which we are trying to look at is that what speed we are considering. Now there are different elements and for different elements different type of speeds may be required to be considered while designing them. So design speed is one of that element and then on the basis of the approach speeds on the basis of the turning speed some of the elements are going to be looked at and we are going to talk about these things in some future interaction. We have to look at the curves, the corners, the radius of the islands which are being provided so as to segregate the movements, the turning paths which are going to be defined on the basis of the steering capabilities on the basis of the turning radii of different categories of vehicles, superelevation, superelevation again on the basis of the speeds we have to look at, the levels of shoulders and the widths being provided if there is a movement and some of part of the vehicle gets out of the edge of the carriage way than if the shoulders have been provided at the same levels and they are wide enough to accommodate that is going to be helpful. Speed change lanes and their lengths because when we say that we are going to change our speed from 80 kilometers per hour and now we are taking a turn and in this turn we are going to move out and at this point the speed is going to be 40 kilometers per hour. So there is a change from 80 to 40. So we have to provide a speed change lane like this. So there is a deacceleration lane here and this length of the deacceleration lane is going to be decided on the basis of acceptable acceleration rate. It cannot be abstract, you cannot make it abruptly a change from 80 to 40. So what should be that length that is also an important thing, channelization for the segregation of different maneuvers, signs and markings to inform the drivers the way they should move in which particular direction, they should move at what time, at what place and all of those things. So there are different things which needs to be taken care of when we are talking about uniformity and simplicity. Then another point is minimum conflict point or area. Now how we can ensure these things? So if we talk about a 4 lagged intersection like this, then there are different movements. The traffic is going this way, the traffic is taking a turn and the traffic is again taking a turn here. So there is a left, right and the straight movement and this is happening from each direction. So when so many maneuvers are happening in this open area, then what you found is that this total area becomes a conflict prone area and we should try to reduce these particular conflict prone area or the specific conflicts which are happening in this intersection. Say we have this 4 lagged intersection and if we have converted into a roundabout. So instead of having the same way as we have just shown here, it becomes in this form. So the things which are happening here is that because of this roundabout that movements are going to happen in the circular motion at this area and when this circular pattern is there, there is a possibility of reducing the conflict points from 32 to 12. So a good reduction in the number of conflict points is there. At the same time, the major area of the intersection has been taken up by this island. So the conflict area is also getting reduced to certain locations and this can be done specifically say on rural highways where we are not interested to stop the traffic and the traffic can keep moving and the roundabouts allow that type of movement. Say we are going for a signalization of an intersection. Now when we go for a signalization, then there is a possibility that on this type of a system, we are saying that we are going straight and we are allowing the traffic to take a free left and similarly, there is a traffic which is coming from the other direction like this and it is also being allowed to take a free left. So at one point of a time, only these maneuvers are being allowed. Now if this is happening, then there is no conflict. So the conflicts have been reduced to minimum or even they have been eliminated in these cases. So this all depends on what are the number of signal phases which have been utilized while designing any intersection into a form of a signalized intersection. Channelization is another thing which helps in ensuring the smooth flows, the more safer flows and at the same time, it allows a different type of maneuvers to take place. So that is being achieved by way of the provision of different islands and the design of these islands will be the point which we have to look at when we are talking about this channelization. There can also be a possibility that so as to segregate the movement of different categories of vehicles, we can also think of prioritizing the crossing traffic. Say we are giving first of all the priority to the bicycle or the priority to the public transport. So there can be a bicycle box at the front of the motorized vehicles. We have seen some of the such diagrams or the photographs previously or we are saying that give way sign that means first of all you have to allow the vehicles which are moving at a cross movement to take their way and then once that will be done, you have an opportunity to move then move inside or the crossroads of the same importance are there, then the priority can be given to the traffic on the right. So that is a rule, thumb rule which can be utilized while looking at how the traffic needs to be allowed to move or converting a four-legged crossing into a staggered crossing. So instead of having a crossing like this, what we are saying is that the crossing has been made in this form. So that is an staggering which can be there and these crossing arms which are there, if they are placed at a distance more than 45 meters, then that is going to be good way of doing it. But when we are talking about this type of a system, say the traffic comes here and this way. So the traffic which is coming from here goes this way and then this way. So this is a right-left stagger or the other way is that it is in this form. So the traffic coming from here goes this way and this way, then this is a left-right stagger. Now right-left that means this is RL. So this is better than LR. Now what may be the reason of this is needs to be looked at. So when we are talking about this, where first of all the right movement is there and then the left movement is there, where spacing at the most absolute minimum is being shown as 15 meters. What happens is that there can be a stop line here. So this is a stop line. The traffic is moving straight in this form as well as straight in this way, two directional traffic is there here. So the vehicles which are coming on this side, they stop here, they find out the way that whenever the opportunity is there, they are going ahead. So as soon as they found that there is an opportunity, the vehicle moving on this side or the vehicle from this side is at a distance away. So this vehicle comes in this position and then it becomes a free left. So the vehicle goes into this direction. So there is a one thing which is a crossing movement and the rest is a simple movement which is happening. But if you talk about the other case, what happens is the vehicle which is moving from this side. First of all, if I make it a bigger one, then what happens is that this can be the possibility. So the vehicle is here at this stop line, then it has to go in this direction. So this vehicle will get here, then it is going to cross, will come in the position here and then it is going to take a turn. At the same time, there is a traffic which is moving in this direction. So first of all, there is some conflict here. Then if it is a two-lane system, then the conflict can come on this side also and then a conflict is coming here. So at this point, this is basically getting merged. At this one, this is weaving which is happening and then it is diverging. And when this vehicle is standing here, so as to wait to take a right turn, there is a possibility of the queuing at the back. So many things are going to happen when you are going for this left-right system rather than a right-left system and that is the reason this one is being assumed to be better than the other one. Then another thing is that when you have to control, as I talked about in the previous one, so we were talking about this right turning traffic. This right turning traffic can be controlled by way of the provision of either a right turning lane or the channelization on the minor road or giving the priority to the right turning lane by way of the signals or there can be a police control. So when we say that it is being done by the right turning lane and the channelization, then it is something like this way. So the traffic, the vehicle which is coming from here gets into this lane and stops here. So this is a right turning lane and then as soon as there is an opportunity, this vehicle is going to take a turn. So here we have a stop condition which is there and if this vehicle is coming from here, then here also we have a stop condition. And right turning lane on a major road will help in preventing rear end collisions. The reason being that the vehicles which are coming from here, now if this vehicle is stopping here, then there is no other vehicle which is going to hit at the back because these vehicles are already having a slower speed. So that is the way we can improve the things. And the provision of left turning traffic, the provision of left turning traffic can be provided in different forms. Say we are coming from here, so there is a road. So what we do is as I have shown you previously too. So this traffic gets into this one. So there is a deacceleration and then it is going out. But in the other case what happens is that the traffic is coming from the minor road, it is going on the major highway. So this comes into acceleration lane and after some time it merges. So we have a deceleration lane and we have a acceleration lane depending on whether it is an exiting or a entering traffic from the main highway to the minor road. So that is another thing we can look at. So this particular one is talking about the various types of conflicts which can be there and then based on the way the provision of the channelizations has been done, the medians has been done and the different movements have been segregated out, what we can see is the continuously we can reduce the number of conflicts say even to 3 in this particular case. So it all depends on how the movements have been controlled and how the things have been done. So we can stop here and we will be continuing from here with our further discussion. Till then thank you and bye.