After completing this lesson the student will

be able to understand the concept of capacity and level of service for highway facilities,

identify the factors affecting the capacity and level of service, the student will be

able to estimate the capacity and surface volumes for different highway facilities. Let us try to understand the concept of capacity. First a formal definition as given by Highway

Capacity Manual 2000. Capacity is defined as maximum hourly rate

at which persons or vehicles can be expected to traverse a point or uniform section of

a lane or roadway during a given time period under prevailing roadway traffic and control

conditions. There are several interesting and important

aspects of this capacity concept as given in the definition so I would like to emphasize

on those aspects. Carefully observe that capacity is defined

as maximum hourly rate and not the volume. The difference in traffic volume and rate

is already known to you. It is the maximum hourly rate. Usually peak 15 minutes is taken and expressed

as equivalent maximum hourly rate so that rate capacity refers to maximum hourly rate. It is also defined or expressed in terms of

persons or vehicles. So it may be in terms of vehicle capacity

or it may be in terms of passenger capacity. Passenger capacity concept is critical particularly

when transit is considered or high occupancy vehicle lengths are considered. In that context person and capacity it is

more meaningful and contextual. Observe that capacity is also defined for

prevailing roadway traffic and control conditions. What is prevailing roadway traffic and control

condition? If you pick up a roadway segment and try to

understand the capacity it is under the given roadway, traffic and control conditions. That means capacity is linked or defined with

respect to a given or prevailing roadway traffic and control conditions. You change the roadway condition you change

the traffic condition you change the control condition then capacity is going to be in

place, you will get different capacities. Also, capacity is defined for a point or uniform

section. This is again consistent with prevailing condition. Uniform section means uniform in terms of

roadway traffic and control condition so along the length of a road at different segments

or at different points the prevailing roadway traffic and control conditions vary so obviously

the capacity also will change. So capacity is again referred to a point or

uniform section. Also capacity refers to maximum flow that

can reasonably be expected, this point is very very important. It can be reasonably expected to traverse

a section. That means we are indicating that capacity

varies both in time and space. Remember that it is included in the definition

that it can be reasonably expected. That means not the single highest flow level

ever expected to occur for a facility rather we are referring to max flow rate a value

that can be reasonably achieved repeatedly not once in a while once in a lifetime you

achieve the highest flow and that you try to refer as capacity, no, that is not the

capacity. Capacity is the max flow rate tat can reasonably

be achieved again and again for a given facility. So capacity is not the single highest flow

level. Therefore isolated observations more than

capacity is nto a contradiction but an expected condition. If you measure traffic flow and express it

in terms of flow rate sometimes you may get even a value which is higher than the prescribed

capacity of that facility under prevailing roadway traffic and control condition. It is not a contradiction to capacity definition. You should not think that since you have observed

the flow more than the prescribed capacity there is something wrong. It is not a contradiction but an expected

condition. With this concept of capacity let us now try

to understand the concept of level of service. Again first a formal definition has been given

by Highway Capacity Manual 2000. It is a quality measure describing operational

conditions within a traffic stream generally in terms of such service measures as speed

and travel time, freedom to maneuver and traffic interruptions, comfort and control. So, it is a quality measure which is describing

operational conditions within a traffic stream in terms of service measures such as speed

and travel time, freedom to maneuver and traffic interruptions, comfort and control. Now six levels of service are recognized,

it is designated as from A to F so in short we say LOS A, LOS B and so on with LOS representing

the best operating condition so now max freedom to maneuver is possible while it is a free

flow condition so obviously Level of Service A is defined as free flow condition and LOS

F is defined for worst possible operating condition that is the forced flow condition

or breakdown flow condition. For each facility type one or more stated

performance measure serves as primary determinant of Level of Service. We have understood the concept of level of

service but how to say that a given facility is operating under this particular level of

service we need some quantity to basis for defining the level of service. The quantitative basis is called as measure

of effectiveness, in short MoE for that facility. Now each level of service represents a range

of operating conditions and driver’s perception for this condition. Whatever may be the measure of effectiveness

it is within a prescribed range we call it a particular level of service. It is in another range we call it another

level of service so each level of service represents a range of operating condition

and the driver’s perception of those conditions. So level of service essentially are stiff

functions under certain it is A then B then C like that. So these are essentially stiff functions each

representing a range of operating conditions. Let us have a look at some of the measure

of effectiveness for different LoS for uninterrupted flow facilities and also for interrupted flow

facilities. For uninterrupted flow facilities say freeways

the measure of effectiveness is density. For different facilities different measure

of effectiveness are defined. For freeways it is density which is normally

expressed as passenger car per lane. How many passenger cars per lane, per kilometer? For multilane vehicles again density is number

of vehicles per lane per kilometer. For two-lane highways two measures of effectiveness

are used. One is average travel speed and the other

one is percentage time spent following because particularly for two-lane highways overtaking

or passing opportunity is a crucial aspect. So often it may happen that when a heavy vehicle

is going all the other vehicles are forced to follow that so what is the percentage of

time spent for following is also a consideration. so it is based on average travel speed and

percentage of time spent following. For interrupted flow facilities signalized

intersection and unsignalized intersection both is control delay, how many seconds delay

per vehicle. We will discuss more about control delay,

what is control delay. For urban streets it is average travel speed

so it is may be kilometer per hour. These are general ideas about MoEs for different

roadway facilities for both uninterrupted and interrupted. It is worthwhile to mention that two similar

facilities with same level of service may differ more than two with different level

of service depending upon where they are in the defined range. Let me try to explain this part. Suppose we are defining LoS in a range, supposing

this is one range, let us call it LoS B and then this is the range for LoS C, so let us

consider two operating conditions here both are defined by Level of Service C so in MoE

values the difference may be much higher but both of them are under the level of service

C whereas let us now consider another operating condition that is here, the difference between

these two points are very less in terms of MoE values but one is the left side of the

boundary and another is may be on the right side of the boundary so in one case it is

the level of service B and another case it is the level of service C. So whatever measure

of effectiveness we have considered you may follow that there are two operating conditions

in the same level of service but MoE values are much different or as compared to another

two operating conditions where you are actually telling them two levels of service but the

difference in MoE is much lesser. Coming to the concept of service flow rate

this concept is again related to level of service. Service flow rates generally represent the

max flow rate that can be accommodated while maintaining level of service. That means for a given level of service range

what is the max flow that can be ……. That means max flow that can be accommodated within

that level of service. Obviously it goes to the boundary condition. So service flow rates may be defined for levels

of service A to E but service flow rates are not defined for LOS F. That is it is obvious

because LOS F represents unstable flow operation or forced flow operation that you cannot really

define the maximum service flow with respect to LOS F. This is a representation of Level of service

in the form of indicating volume condition. You are familiar with the shape of this curve

it is speed volume car when the volume is practically significant in the lean hours

in the morning or midnight you may find less number of vehicles which are traveling at

a very high speed and then as vehicle volume or flow increases there is a drop in the speed

so it may not be so steep but this is a conceptual depiction so you can define this whole operative

range into different levels of service. This is level of service A, this is operating

zone B, this is C, this is D, maximum service volume with respect to E, this is nothing

but the capacity and then this whole shaded portion represents unstable flow. Therefore you can clearly understand why we

don’t define service volumes with respect to Level of service F. this is a conceptual

representation of level of service in speed volume flow. Now coming to the factors affecting capacity

and level of service there are several factors under some broad categories. First we have the roadway factors. We have the land width: according to the Highway

Capacity Manual a lane width of 3.6m is considered ideal. So, if the lane width is varying or is lesser

than 3.6m obviously the capacity will be lesser or the capacity will decrease. Lateral clearance: constructions such as retaining

walls, abutments, signposts, light posts, parked cars etc is located closer than 1.8m

from the edge of traffic lane means that lateral

clearance is less than 1.8m so this obstruction will show different capacities and therefore

the capacity will reduce. The next roadway factor is width of shoulder:

the narrow shoulders reduce capacity, effective width of traffic length as the vehicle travels

towards the pavement. Therefore the narrow shoulder also affects

the capacity. Grade: also affects the speed of vehicle particularly

if you have substantial proportions of commercial vehicles or heavy vehicles. Therefore on grades the presence of heavy

vehicles together may really influence the capacity to a large extent. Presence of intersections also influences

capacity because intersections restrict the free flow of traffic and thus adversely affect

the capacity. Alignment also influences the capacity particularly

sight distance. If adequate sight distance is not available

then that will affect the safety as well as the capacity on the road. Surface condition also will affect capacity. If the road surface condition is very poor

then obviously the speed will be reduced because the vehicles cannot travel at higher speeds

and therefore the capacity will get lesser with roads with bad surface conditions. So these are all namely roadway factors which

affect the capacity. There are traffic factors, these are a set

of factors which again affect the capacity. Particularly the percentage of commercial

vehicles are very important. If there are more commercial vehicles in percentage

then the capacity is going to be affected. Larger commercial vehicles like trucks and

buses influence the traffic stream in two different ways. First they occupy more space than passenger

cars because they are bigger vehicles but they are distinctly different in terms of

acceleration and deceleration….. so that is another major aspect as to why the percentage

of commercial vehicles affect capacity. They have more operating capabilities than

passenger cars particularly with respect to acceleration and deceleration and the ability

to maintain speed in upgrade conditions. That is why traffic factors are affected. Directional distributions also play an important

role particularly for two-lane highway facilities. both for upstream and down stream traffic

they use the same road. A two-lane road is not a divided road therefore

the capacity is also influenced by the directional distribution of the …… Let us look at some of the main points as

mentioned in the Indian Roads Congress guidelines. As per IRC it is not advisable to design the

width of a road for a traffic volume equal to its capacity. Obviously there is a reason for that because

if the facility is designed for level of service E then there is no doubt that the traffic

volume can be accommodated but with a poorer performance. The speed will reduce, the density will become

very high, the freedom to maneuver will come down drastically. Therefore the quality of traffic operation

will not be up to the desired level. Therefore the facility is not designed with

respect to the service volume with respect to level of service E for capacity. IRC recommends that under normal conditions

level of service E may be adopted for rural roads and facility may be designed for level

of service C for urban roads. Accordingly instead of capacity we can take

level of service B or level of service volumes while designing rural and urban roads. In rural highways instead of hourly volume

daily traffic volume is adopted for design. So IRC manual guideline suggests that daily

volume is adopted for design of roads. Obviously the daily hourly volume is derived

assuming 8 to 10% of the daily traffic has peak hour time. So there is relation and th… hourly volume

is also considered but IRC quote says that capacity is in terms of daily volume but not

in terms of hourly traffic. Design service volume considered for design

purpose is expected volume at the end of design life. That means when we are going for road infrastructure

planning we plan the facility or we consider economic life period. Normally for a road project it is 20 years

so the traffic volume what we are taking and comparing with maximum service volume with

respect to level of service B for rural highways that traffic volume is not the present traffic

volume but the projected traffic volume at the end of design life. Therefore at the end of design life what is

going to be the traffic volume we take that and accordingly we decide what will be the

lane requirement or capacity requirement. Obviously this can be computed by projected

or forecasted traffic by projecting the present volume at an appropriate traffic growth rate. so we take the present traffic volume and

estimate the ADT Average Daily Traffic, we take into consideration the seasonal variation

so we estimate AADT Annual Average Daily Traffic, use suitable growth factors to project this

base year or the current year Annual Average Daily Traffic to forecast traffic at the end

of design. Hence the traffic growth rate should be established. It should be done after carefully studying

the past trends and potential for future growth of traffic. So there are different methods of estimating

traffic growth rate. We are not going into the details. But the basic steps are estimated by …. traffic

use suitable growth factor, forecast base year traffic. The growth factors are estimated by looking

at the past traffic trends and also by looking at the future strategy of development in the

influence area of the project. This is an example of the design service volumes

as given in IRC manual. We consider three types of terrain: plain,

rolling and hilly. Again we divide it based on the curvature

low or high curvature and in which terrain condition. Curvature is defined based on degree per km,

the horizontal deviation of the curvature in terms of degree per km so they define accordingly

whether it is low or high and for each type of terrain with each type of curvature the

designs are given in terms of passenger car unit per day. So it is so many cars per day. So it is daily volume that is considered. The capacity of two-lane road can be increased

about 15% by providing paved and surface shoulder of at least 1.5m width on either side. This is again an IRC provision. So you provide paved and surface shoulders

of at least 1.5m width on either side then the capacity of the service volume values

may be increased by 15%. Where the shoulder width or carriage width

on a two-lane road is restricted there will be certain reduction in capacity so IRC guidelines

also gives a table to calculate the factors like what is the usable shoulder and what

is the lane width according to these factors that may be obtained or taken from the table

given in IRC quote and accordingly this capacity reduction may be done. Highway Capacity Manual approach: The analytical

methods in HCM attempt to predict the maximum flow rate for various facilities at each level

of service except LOS F as already stated because LOS F represents unstable flow. Each facility has five service flow rates

LOS A to LOS E representing one for each level of service that is A to E. Service flow rates are discrete values whereas

level of service represents a range of conditions. Level of service represents a range of condition

but service flow rates are discrete values. To know the maximum traffic flow rate is only

possible by maintaining given levels of service. Hence these are essentially boundary values. Therefore obviously service flow rates are

discrete values and level of service represents a range of conditions. Service flow rates are at the maximum for

each level of service and they effectively define the flow boundaries between levels

of service. So obviously as I indicated these are the

different levels of service. So the maximum level of service with respect

to the flow rate is this one and again here and again here. Hence essentially these are maximum flow rates

for each level of service. They effectively define the flow boundaries

between levels of service. Now an understanding about the base condition

is important. Procedures in HCM provide formula sometimes

in a simple tabular form or a graphical representation for a set of specified standard conditions

which must be adjusted to account for prevailing conditions. There are prevailing conditions which are

not ideal or which are not the base conditions. HCM defines certain base conditions and then

for a prevailing condition suitable correction factors may be applied to represent it or

to estimate or assess the level of service for a given condition which is the prevailing

roadway traffic and control conditions. The standard conditions used in HCM so defined

are termed as base conditions. Base conditions assume good weather, good

pavement conditions. Assume that users are familiar with the facility

that there are no heavy vehicles in the traffic stream and there is no impediment to traffic

flow. Hence through all this the base condition

or the ideal condition is defined. Let us have a re-look for the base conditions

for uninterrupted flow facilities. Base is nearly an ideal condition. Lane width of 3.6m Clearance of 1.8m between

the edge of the travel lane and the nearest obstruction or the object and the roadside

or in the median Free-flow speed of 100 km/h for multilane

highways Only passenger cars in the traffic stream

that means no heavy vehicles are present in the traffic stream

It is level terrain, there is no no-passing zones that means no restrictions for overtaking

or passing on two-lane highways. These are the conditions which are considered

for defining these things. Also it is assumed that no impediments to

through traffic due to traffic control or turning vehicles. So, if there are left turning or right turning

of vehicles or other kinds of obstruction for movement of traffic that will again affect

the capacity or division of the base condition. So in base condition assume that there is

no impediment through traffic or traffic control or turning vehicles. Three types of facilities are described in

Highway Capacity Manual as follows: Basic freeway

Multilane highway Two-lane highway We will discuss about these three types of

facilities and how the level of service can be calculated for prevailing roadway traffic

and control conditions. First is we have the Basic Freeways. HCM defines free flow speed as base condition. This is denoted as BFFS Base Condition Free

Flow Speed and also Free Flow Speed FFS under prevailing conditions. So FFS is the mean speed of passenger cars

measured during low to moderate flows. So it is essentially free flow. Now, it is possible that FFS may be measured. If speed measurement of FFS Free Flow Speed

is not possible then this can be estimated indirectly on the basis of the physical characteristics

of the freeway segment that is studied. Let us look at this and find out how it is

possible. Free flow speed equal to free flow speed under

base conditions minus reductions due to a number of factors. For freeway segments the corrections are applied

for adjustment of lane width, adjustment for right-shoulder lateral clearance following

U.S. convention of driving, adjustment for number of traffic lengths, adjustments for

interchange density. So these are the factors which may affect

free flow speed and which may cause the FFS to be different from BFFS. Therefore appropriate corrections are …. for

adjustments are lane width, adjustments for right-shoulder lateral clearance, adjustment

for number of lanes and adjustment for interchange density. Now how to get these values. For further discussions the figures, charts

and tables are given. So one can calculate these adjustment factors

using those prescribed values and accordingly calculate FFS the Free Flow Speed for the

prevailing condition. Next is determining flow rate. The hourly flow rate must reflect the influence

of heavy vehicles. Because in the ideal condition or the base

condition we assume that there is no commercial vehicles or heavy vehicles. So the hourly flow rate must reflect the influence

of heavy vehicles, the temporal variation of traffic flow over an hour and also the

characteristics of the driver population. These are the three factors we considered

to estimate the determining flow rate. Here Vp is the 15 minute passenger car equivalent

flow rate, this we want to calculate. We know V which is the hourly volume, V by

peak hour traffic so this is to account for temporal variation of traffic flow over an

hour into n which is the number of traffic lanes, fHV to account for the influence of

the heavy vehicles and fp to account for the characteristics of driver population. Again what should be the value of fHV, fP

and peak hour factor then with the help of HCM we can get an idea about this flow. Peak-hour factor is basically to represent

the variation of traffic flow within an hour. Observations of traffic flow consistently

indicate that the flow rates are found in the peak. A 15 minute period within an hour is not sustained

throughout the entire period and that is why we need to use the peak-hour factor. Normally on freeways the peak-hour factor

values range from 0.80 to 0.95. So if the peak hour factor is known then the

same value may be used or suitable values may be adopted at a range of 0.80 or 0.95

matching the conditions. Heavy vehicle adjustment factor what I have

indicated here this fHP component we are now going to discuss a little bit more about this

fHP or the heavy vehicle adjustment factor. Freeway having mixed traffic volumes must

be adjusted to an equivalent flow rate expressed in passenger car per hour per lane. This adjustment is made using the factor fHV

for heavy vehicle and how we calculate fHV is by using this equation. Now here in ET and ER two different types

of heavy vehicles are normally considered in highway capacity manual. It is the passenger car equivalent for trucks

and buses, ER is passenger car equivalent for recreational vehicles. Recreational vehicles are also heavy vehicles

and on certain routes the recreational vehicles do operate so if there are recreational vehicles

accordingly then its ER value is to be taken. Again to find out what should be the value

of ER and ET so further discussions are made in Highway Capacity Manual and one can pick

up some of these values. Now PT and PR are proportion of trucks and

buses, and PR is the proportion of recreational vehicles in the total traffic stream. Therefore once we know all these PT and PR

the ET and ER values may be obtained from HCM manual, we can calculate the fHV the factor

which is to be used for heavy vehicle adjustment. Now determining LOS we estimated field measured

free flow speed so we have mentioned how we can calculate FFS and the appropriate free

flow curve of the same shape of the typical curves is constructed then on the basis of

the flow rate and the speed of curve and the average speed of the passenger car is read

on the y axis. Let us take this one as an example. So these are different speed flow curves which

are given. This is a line which is representing a free

flow speed of 120 km and this is another line which is representing a free flow speed of

110 km, this is another operation which is representing a free flow speed of 100 km and

this is another which is representing 90 km. Suppose once we calculate our free flow speed

for the prevailing condition say 95 km so what we are trying to indicate is you have

this line for 100 km, you have this line for 100 km so in between the red line indicated

may be considered for representing a free flow rate of 95. Therefore we know that we can construct this

line or the speed volume relationship. Then if we know the speed say it is 1700 or

1800 then accordingly we can understand the speed that can be read. So first we construct this curve and then

for a given volume we calculate the speed. Now the speed is known and the volume is known. So once the speed is known and the volume

is known we can calculate the density and volume by speed, so the speed we have taken

on y axis, VP is also known, so now we know VP and S where S is the density. Now remember that for freeway segment measure

of effectiveness is density. So that is why we are calculating density. Now once the density is known then for a given

condition we can refer the appropriate table and estimate the ………and what will be

the service volume corresponding to that length. Now further discussions are available and

the actual tables are given in terms of exhibit….. and those are available in highway capacity

manual. So LOS of basic freeway segment is determined

by comparing the calculated density with the density range given in exhibit and the service

volumes also can be estimated. These are some of the details about freeways. Now coming to multilane highways the approach

is similar whatever we have discussed for freeways but the only thing is the values

and corrections and the factors are different for multilane highways. For example, here also appropriate corrections

are necessary to obtain free flow speed under prevailing conditions either from the base

conditions or ideal conditions. But here also we have applied similar corrections

but look at the corrections they are different from what we applied for freeways. Here we are applying corrections or adjustments

for lane width, we are applying adjustment for lateral clearance, we are applying adjustment

for median time and also we are applying adjustment for excess points. Earlier the access point adjustment was not

there. So the approach is similar but actual adjustments

which are applied are different. So again suitable values may be given and

further discussion are given in highway capacity manual and you can get the values for fLW,

fLC, fM and fA and accordingly you can calculate FFS for multilane ……… Basically two-lane highways are classified

in two classes for analysis purpose such as class I and class II because two-lane highways

are different from multilane highways and freeways. Class I is highways on which motorists are

expected to travel at relatively high speeds. So here speed is a major consideration because

motorists are expected to travel at relatively higher speeds so that is one classification. Class I includes two-lane highways that are

major inter-city roads where the speed will be higher. Primary arterials connecting major traffic

generators differ, the speed is a major concern for the daily commuter routes which are normally

used for commuting purposes or primarily links in state or national highway networks. These are the roads which are normally included

under classification. Class I facilities most often serve long-distance

trips, they are important and that is why speed is important or provide connecting links

between facilities that serve long-distance trips. Now coming to class II these are two-lane

highways on which motorists do not necessarily expect to travel at high speeds. Generally this type of road serves as access

route to class I facilities and motorists necessarily

do not travel at high speeds. They normally serve as scenic or recreational

routes that are not primary arterials. Therefore very high speeds are not expected. Hence speed is not the major consideration. Therefore in class II facility most of them

relatively short trips, the beginning and end portion of longer trips and trips for

which sightseeing plays a significant role. So that way class I and class II routes are

different. Now the remaining task is easy. It is now easily understood or we can get

convinced that in class I highways efficient mobility is paramount, and LOS is therefore

defined in terms of both percent time spent following and average travel speed. On class II highways the mobility is less

critical and therefore LOS is defined only in terms of percent time spent following without

consideration of average travel speed. So that is why LOS is different, the measure

of effectiveness is different on the class I and class II type two-lane highways. Question Set: Some of the questions for you to answer: Define capacity and level of service Describe different traffic factors affecting

capacity How Free flow Speed (FFS) is estimated from

Basic Free Flow Speed from multilane and two-lane highways, what are the corrections required. What are the MOEs considered while estimating

the capacity of two lane highways? Try to answer these questions. Now I will try to answer the questions of

lesson 2.4. What are the practical limitations of using

the radar meters? You know that in some cases you may have to

obtain license if the frequencies are within the government regulated frequencies and wave

lengths. Accuracy may vary. It is difficult to conceal the radar meter

so the drivers tend to slow down. Accurate measurements are obtained only when

the radar wave is reflected directly along the axis of the vehicle movement which is

practically difficult. Therefore adjustment requires a good measurement

of the angle of wave deflection. Different ways of presentation of spot speed

data: One is in the tabular form where you decide

the upper limit and the lower limit and the middle value, observed frequency and the percentage

frequency and the cumulative frequency and the frequency distribution table. You represent it using the Frequency Distribution

Curves, you also represent it using the Cumulative Frequency Distribution Curve. How is the travel time studies carried out

using the average car technique? Remember that in the average car the driver

is instructed to drive at an approximate average speed of the traffic stream and this is particularly

suitable for multilane facilities where it is difficult to follow the floating curve

method. So there the average car technique is more

appropriate considering safety and conveyance. Now we record a number of data here and the

travel time in each direction separately and for each direction of travel the number of

overtaking vehicle, the number of overtaken vehicle and number of vehicles from opposite

directions etc are recorded and then appropriately they are adjusted to calculate the average

speed or the representative stream speed time and once we calculate the ……using the

appropriate calculations then you can calculate V and Q. Finally let us look at what is the

total control delay at intersection. Total control delay generally includes time-in-queue

delay and time losses due to acceleration from and deceleration to ambient speed. Suppose there is no intersection that the

vehicle will go so the delay is due to the time-in-queue, the amount of time the vehicle

is in queue so that is the delay which is one component and another component is from

the normal speed once the speed is reduced because of this intersection there is deceleration

so it is from ambient speed to lower speed following deceleration and again from stop

condition to ambient speed using acceleration. So it is control delay the time-in-queue delay

plus time losses due to deceleration from acceleration to ambient speed.