Lecture – 6 Highway Capacity and Level of Service


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.

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