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HCM method of arterial performance measurement involves seven steps which aim
to compute 'average travel speed' of arterial to measure the Level of Service.
These seven steps are as follows,
- Establish arterial to be considered
- Determine arterial class by free flow speed
- Define arterial section
- Compute running time
- Compute intersection approach delay
- Compute average travel speed
- Estimate the LOS.
The above flow chart shows the steps to determine LOS in a schematic form.
Further in this section we are going to discuss these seven steps in detail.
Establishing the arterial is the very first step in the process of determining
the LOS.
In this step, an engineer has to define arterial segment or entire arterial
whose LOS is to be determined.
Arterial may be established by arterial class, its flow characteristics and
signal density.
Arterial class may be defined as per its free flow speed as explained in step 2
as follows.
Free flow speed is the speed on the arterial which most of the drivers choose
if they had green indication and they are alone in the direction of movement
are not the part of platoon) but have to be conscious about all other
prevailing conditions.
(e.g. Block spacing, contiguous land use, right of way, characteristic,
pedestrian activity, parking, etc.) Free flow speed should be measured at just
the time when the entire factors are present except for the prevailing traffic
levels and red indication.
An arterial can be classified on the basis of its free flow speed as explained
under the section design based classification and combined classification .
The following table 3 can be used to determine the arterial class.
Table 1:
Range and typical values of FFS for different arterial classes
| Free flow |
Arterial Class |
| speed (kmph) |
I |
II |
III |
IV |
| Speed range |
90 to 70 |
70 to 55 |
55 to 50 |
55 to 40 |
| Typical value |
80 |
65 |
55 |
45 |
After determining the arterial class it is required to be more specific about
the particular section of an arterial for which LOS is to be determined.
The arterial section may be mid block or intersection.
Generally signalized intersection is taken into account to determine
intersection approach delays which are further required to determine level of
service.
There are two principal components for the total time that a vehicle spends on
a segment of an urban street.
These are running time and control delay at signalized intersections.
To compute the running time for a segment, the analyst must know the street's
classification, its segment length, and it's free flow speed.
Arterial running time can be obtained by Travel time studies, information of
running times from local data and intersection delays etc.
Intersection approach delay is the correct delay which is to be used in
arterial evaluation.
It gives consideration not only for absolute stopped delay but also for the
delay in retarding the vehicle approaching at signal for stopping and
re-accelerating on starting of green.
It is longer than the stopped delay.
This can be related to intersection stopped delay and is computed by,
 |
(1) |
where, D = intersection approach delay (sec/veh), and d = intersection stopped
delay (sec/veh).
Delay at intersection approach is of special interest because it is a Measure
of Effectiveness (MOE) used to quantify LOS.
To determine intersection approach (or control) delay it is necessary to
calculate stopped delay which is discussed below.
Stopped vehicles on intersection are counted for intervals of 10 to 20 seconds.
It is assumed that vehicles counted as 'stopped' during one of these intervals
will be stopped for the length of the interval.
Measuring the stopped delays involves following steps.
- Maximum extent of queue length on intersection approach during the study
period must be observed in advance (observer must be able to count all stopped
vehicles in the longest possible queue).
- Count intervals are set at 10, 15, or 20 seconds stopped vehicles within
the queuing area observed and recorded at each interval.
- Discharge volumes are separately counted for the study period.
In an intersection the following data was observed for stopping times for
vehicles as tabulated in table 4.
Calculate intersection approach delay for the given data set.
Total exiting vehicles: 100.
Table 2:
Data observed at an intersection for stopping vehicles
| |
Seconds into minute |
| Minute |
0 sec |
15 sec |
30 sec |
45 sec |
| 5.00 pm |
2 |
4 |
1 |
3 |
| 5.01 pm |
4 |
5 |
3 |
0 |
| 5.02 pm |
6 |
3 |
2 |
1 |
| 5.03 pm |
2 |
5 |
4 |
3 |
| 5.04 pm |
4 |
2 |
6 |
4 |
| 5.05 pm |
5 |
4 |
1 |
1 |
| 5.06 pm |
1 |
2 |
5 |
5 |
| 5.07 pm |
4 |
3 |
3 |
3 |
| 5.08 pm |
2 |
5 |
2 |
2 |
| 5.09 pm |
3 |
1 |
4 |
2 |
| Total |
33 |
34 |
31 |
24 |
Total of stopped-vehicle counts (density counts) for study sample is:
33+34+31+24=122 veh.
Each of the vehicle interval is 15 seconds.
Aggregate delay for the 10 minutes study period is,
122 15 sec=1830 veh-sec.
Average stopped delay per vehicle for study period of 10 minutes is,
1830/100 =18.3 sec per vehicle.
That is, d=18.3 sec per vehicle.
We use this in the first equation.
So, intersection approach (or control) delay D
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