# Lecture – 13 Intersection Sight Distance – II

Intersection Sight Distance part II. In the last lesson we discussed about the need for providing Intersection Sight Distance, why it is necessary to provide Intersection Sight Distance. We also discussed the concept of sight triangles particularly approach sight triangles and departure sight triangles. We also discussed about different types of intersection control that affect required Intersection Sight Distance particularly different intersection control as considered by AASHTO American Association of State Highway and Transport Officials We also discussed about the basis for estimating required sight distance Intersection Sight Distance for two particular types of intersection control namely no control and stop control on minor road So we have seen that how or what should be the basis for estimating Intersection Sight Distance for these two types of intersection controls In today’s lecture after completing this lesson the student will be able to understand the basis for estimating required Intersection Sight Distance for four types of intersection control considered by AASHTO like case C, case D, case E and case F We have already discussed about case A and case B. So today we shall take up case C that is intersections with yield control, case D intersections with traffic signal control, case E intersections with all ways stop control and case F left turns from minor roadways, what should be required intersection in that case Student will also be able to understand IRC recommendations Indian Roads Congress recommendations about Intersection Sight Distance for various types of intersection control Case C that is yield control on minor road: This is a type of intersection where yield sign is used on minor road and drivers approaching yield sign are permitted to enter or cross the major road without stopping if there are no potentially conflicting vehicles on major road

intersections with no control values for distance were recommended by AASHTO for different design speed because the assumption is vehicles decelerate to fifty percent or sixty percent of the minor road design speed. Obviously the required sight distance is a function of the approach speed So in this case both the length and the time required to cover that distance they are given as a function of the designed speed for the minor road. Thus the table values are given in AASHTO Travel time applies to a vehicle that slows before crossing the intersection but does not stop This is a basic difference between intersection with stop control and intersection with yield control Now here for design purpose it is assumed that vehicles will slow down, they will reduce their speed following certain deceleration and they will reduce speed up to sixty percent of the approach speed for the minor roads but the vehicles will not stop. So they will reduce the speed but they will not stop, for that condition the required time is given and we are referring that here as ta approach time Now corrections on length of time of minor road approach is required based on approach grade This is again similar to what was done for case A minus 3% approach grade along the minor road. there is no need for any correction but from minus 3 to minus 6 correction factors will vary in the range 1.0 to 1.2 depending on the grade and the actual design speed Similarly the correction factor for plus 3 to plus 6% grade the value will vary from 0.9 to 0.1. Again depending on grade and design speed this is similar to what was used for case A to apply corrections based on grade So a similar correction is applied here also Let us see how the length of leg along the major road can be calculated Let us try to understand the basis for this, what is the basis for doing that The basis is sufficient travel time for major road vehicle to allow minor road vehicle to travel from decision point to intersection and then cross major road So there are essentially two components of time. one is the time required to travel from decision point to the intersection and then time required to cross the major road. So the total time required could be estimated and the required length along the major road can be estimated accordingly So if tg is the total time required then tg is expressed as ta plus w plus La divided

by 0.167 V minor where V minor is the design speed or mid block speed for minor approach or the minor road. Now ta is the time, this component is the time required to travel from decision point to the intersection Just now we have discussed about this time component, now plus the time required to cross the intersection this is estimated as the width plus the length of the vehicle. So w is the width of intersection and La is the length of the design vehicle. Do remember that this is again a function of the design vehicle so it will vary depending on the design vehicle Now this is the total distance that is required to be crossed divided by the speed Now normally V is expressed in kilometer per hour so the conversion factor is normally 0.278. In this case carefully note that this factor is 0.167 Now why it is point one six seven? It is because it is assumed that vehicles will reduce speed up to sixty percent of the midblock speed so sixty percent of that speed Therefore the correction factor becomes 0.6 multiplied by 0.278 so that is approximately equal to 0.167. So it becomes the effective speed after reduction of speed to sixty percent So now we can calculate, what will be the time required tg? Once this time is known the length of the leg along the major road can be estimated like this; 0.278 V major remember that it is V major multiplied by tg the time gap Now here 0.72 is the normal conversion factor because we are using V in terms of kilometer per hour Now in the previous page we have seen how this tg can be calculated So once this total time gap is known what is the required length based on the design speed of the major road can be calculated Now remember that value of tg should equal or exceed should equal or exceed the appropriate travel time for crossing the major road from a stop controlled approach A similar approach was also used for stop controlled intersection whatever was the value for t or the time, in that case in this case the value should equal or exceed that time value Now let us refresh our memory, what was the actual time gap? For stop control the time gap was 6.5 seconds to 10.5 seconds depending on design vehicle type, 6.5 seconds for car and higher values for commercial vehicle type So when we are calculating tg using the earlier formula that is this page tg then the calculated value if it becomes lesser than this time gap that is say for car it is 6.5 then we must take this value whatever is recommended for stop control approach For example for car if we find tg is less than 6.5 then while calculating the length of leg along major road we shall assume tg as 6.5. Similarly a different value should be assumed for other design vehicle If the major road is a divided highway with

median which is wide enough to store the design vehicle in that case the approach should be slightly different Because till now we discussed about undivided roads. If it is a divided road and median width is sufficient to store the design vehicle then only crossing of the near lanes needs to be considered. Because near lanes crossing need not be taken into consideration along with near lanes as the design vehicle can stop using the wide median which can accommodate the design vehicle So we shall consider crossing of near lanes only and then the vehicle can be stopped in between using the median Therefore a departure sight triangle for accelerating from a stopped position in the median should be provided because the vehicle may stop in the median. So from stopped position obviously it will be a case for departure sight triangle So a departure sight triangle for accelerating from a stopped position in the median should be provided Now let us consider the case two that is left and right turns from minor roads So in this case also we have to see the length of leg along the minor road and also the length of leg along the major road Length of leg along the minor road: AASHTO recommends the length as 25 m a fixed value. This 25 m is based on the assumption that drivers making left and right turns without stopping remember this part without stopping will slow to a turning speed of 16 km/hr That means vehicles which are taking turn either to the left or to the right they will further reduce speed which will be acceptable or comfortable for making the turns and that speed is 16 km/hr. So considering this accept the length of leg along the minor road is recommended as 25 m Now let us see how the length of the road along the minor road can be calculated So length of the leg along the major road This approach is again similar to stop control intersection but with some modification Now let us see first what was the recommendation for left turns at stop control intersection The time gap was recommended as 7.5 seconds to 11.5 seconds depending on design vehicle Obviously these values are for two lane highway with no median and minor road grade 3% or less Now this value was seven point five second for car and higher value for heavier design vehicle Hence that was the time gap recommended for left turns at stop controlled intersection Here minor modification is done on these design time The time gap here is increased by 0.5 seconds So earlier value for stop controlled intersection

was seven point five to eleven point five In this case it is eight seconds to twelve seconds again depending on design vehicles and this range is for two lane highway with no median and minor road grade three percent or less So here half a second more value is assumed as compared to what was taken for stop controlled intersection. There it was 7.5 seconds to 11.5 seconds. In this case it is 0.8 seconds to 12 seconds Now let us try to understand the basis for this increment in time by half second. Why it is increased? It is found that minor road vehicle needs 3.5 seconds additional time to travel from decision point to the intersection for yield control approach This component was not necessary for stop controlled intersection. So this is the additional component which is required for yield control approach Now at the same time the acceleration time after entering the major road is three second less for a yield sign rather than or as compared to that for a stop sign So here it is 3.5 seconds less. Now why it is less? Because for yield controlled intersections it is assumed that drivers will reduce speed substantially but they will not stop. At least for the design consideration vehicles are assumed to travel at a slow speed but vehicles are not stopped whereas for stop control approach vehicles are required to stop. Therefore the acceleration time will definitely be higher for stop control approach because vehicles will start from zero speed, in this case it is not zero speed so there is easy saving of three seconds So there was additional time requirements of 3.5 seconds and there is a saving of 3.5 seconds So as a net result it is half a second more time that is required for yield control intersection That is why the time gap is taken as eight seconds to twelve seconds depending on the design vehicle For left turns on two-way highway with more than two lanes 0.5 seconds should be added if the design vehicle is passenger car and for each additional length from the left in excess of one that is to be crossed by the turning vehicle So I have mentioned it that the values which were mentioned earlier 0.8 seconds to twelve seconds they are for two lane highway. So if a more wider highway is considered for left turns then corrections should be applied depending on design vehicle and the number of lanes that is to be crossed in excess of one by the turning vehicle So that value is 0.5 second for passenger car and a different value is suggested for other design vehicles For right turns of course no such adjustment is necessary Departure sight triangles like those provided for stop-controlled approaches should also be provided for yield-controlled approaches This is because of the fact that if there is a conflicting vehicle then vehicles approaching

from minor road also may have to stop. So if they stop then the departure sight triangle will be an appropriate consideration So departure sight triangle like those provided for stop controlled approaches should also be considered Normally no specific check for departure sight triangles at yield controlled intersections is needed as approach sight triangles for turning maneuvers at yield control approaches are larger So if we design sight distance considering yield control approach and considering the left turn and right turn without stopping then the required sight distance whatever will be obtained will normally be larger than the stop controlled requirement. So no specific check is normally required If sight distance sufficient for yield control is not available then one may recommend or one may explore the possibility of using stop sign instead of yield sign because then it will be a stop controlled intersection and the required sight distance will be less as compared to yield control intersection Further at locations where recommended sight distance cannot be provided one should explore the possibility of installing regulatory speed sign or other traffic control devices on the major road Remember that we are talking about on the major road to reduce approach speed of vehicles from major road because you have seen that for a given time gap the actual length required will depend on the design speed So, if a regulatory measure is taken to reduce the speed either by regulatory speed sign or with the help of other traffic control devices then the required sight distance along the major road can be reduced or can be minimized Now let us see the case D that is traffic control or traffic signal control, intersections which are controlled by traffic signal In this case the consideration for sight distance is based on the fact that the fast vehicle stopped on one approach should be visible to the drivers of the first vehicle of all other approaches In addition for left turning vehicles left turning vehicles we are saying considering US convention of driving. In Indian condition this is actually the right turn So left turning vehicle should have sufficient sight distance to select gaps in oncoming traffic and complete left turn. This is the additional consideration that may be taken No other approach or departure sight triangles are needed for signalized intersections In fact signalization may be an appropriate crash countermeasure for higher volume intersections with restricted sight distance and those which have experienced intersection related problems and crashes Now there is another special consideration for signal intersection or signal control they are as follows: If the traffic signal is to be placed on two-way flashing operation, what you mean by two-way flashing operation is like this Flashing yellow on the major road approaches

and flashing red on the minor road approaches under off-peak or nighttime conditions If this is the operating condition then this is necessary to provide appropriate departure sight triangles for case B for the minor road approaches because vehicle may stop. So in that case whatever considerations we have discussed for case B that is for stop control approach intersection with stop control on minor road that is to be applied Case E: All-way stop control: In this case also like signal control the fast vehicles stopped on one approach should be visible to the drivers of the fast vehicle stopped on each other approaches. There are no other sight distance criteria applicable for intersections with all-ways stop control In fact all-way stop control may be the best option at a limited number of intersections where sight distance from other control types or sight distance required for other control types cannot be attained So in those cases in a limited number of intersections one can go for all-way stop control for simplicity and for enhanced safety in terms of the sight distance availability Case F: Left turns from major road: Here we are talking about locations where vehicles on major road are permitted to take left turns across opposing traffic Remember that again we are using US convention of driving so the vehicles are permitted to take left turns across opposing traffic If this is the vehicle which is traveling along the major road they have to take left turn like this against this opposing traffic Therefore left turn driver needs sufficient sight distance to decide when it is safe to take left turns across the length used by opposing vehicle and sight distance design should be based on the left turn by a stopped vehicle Now carefully observe this word design Design should be based on a left turn by a stopped vehicle. Now why it is stopped vehicle? The reason is, if vehicle is in motion then it will normally require a lesser sight distance as compared to a vehicle which is stopped Therefore for design consideration it is considered that a vehicle is taking left turn and that vehicle is a stopped vehicle So if the available sight distance is sufficient to satisfy this requirement then automatically it will satisfy the requirement of those drivers which will take left turn without stopping That’s why it is based on a left turn by a stopped vehicle

The sight distance along the major road to accommodate left turns is the distance traversed at the design speed of the major road in the traveled time for design vehicle Whatever traveled time is required for the design vehicle to complete this left turn that much time whatever distance is covered by a vehicle which is traveling along the major road in design speed will give us the length or the required sight distance along the major road So this time gap as per AASHTO is 5.5 seconds to seven point five seconds, that is the time gap which is recommended by AASHTO based on the field observations And this value five point five seconds to seven point five seconds varies depending on the type of design vehicle So for passenger cars it is five point five seconds for heavier vehicles commercial vehicles if they are taken as design vehicle then the required time is higher and it may go up to 7.5 seconds So whatever is the distance covered along the major road if major road speed is V major then 0.278 into V major into this appropriate time 5.5 to 7.5 depending on the design vehicle that will give us the required length Let us see there are a few more aspects For left turning vehicles whatever values we have mentioned in the last slide 5.5 seconds to 7.5 second are actually for two-lane road So for turning vehicles or left turning vehicles that cross for more than one opposing length So point five seconds should be added for passenger car and a separate value is recommended for other vehicle types design vehicle for each additional length to be crossed Obviously if it is a wider road more number of lengths are to be crossed so therefore more time is required. So AASHTO recommends that 0.5 seconds for passenger car should be taken for each additional length to be crossed Now if Stopping Sight Distance has been provided continuously normally throughout the length of the highway Stopping Sight Distance is provided Therefore if Stopping Sight Distance is provided at all points along the highway and if sight distance for stop control or yield control has also been provided for each minor road approach then available sight distance will generally be adequate for left turns from the major road. So separate consideration or separate checks may not be necessary Therefore if throughout the length of the road Stopping Sight Distance is available and at all minor intersections with stop control or yield control appropriate Intersection Sight Distance is available then normally it is not necessary to check for the requirement of sight distance for left turning traffic because normally the sight distance should be adequate for this kind of maneuvers With this we have completed our discussion about all six different types of control considered in AASHTO and the basis for Intersection Sight Distance for each control type