VEHICLE DELAY STUDY

VEHICLE DELAY STUDY
1032交通調查分析
http://ocw.knu.edu.tw/km/1145
Delay definition




Practically all traffic analysis tools produce a
performance measure called “delay”, but tools vary
widely in the definition and computation of delay.
A measure most directly related to Driver’s
experience.
Delay is generally defined as the excess time spent on
a road segment compared with the time at a target
speed that represents a zero-delay condition.
The target speed is the speed at which a specific driver
prefers to drive.
Factors that affect intersection delay(1/3)

Roadway conditions
 Number
of approach lanes
 Lane width
 Grades
 Intersection control type
 Channelization
 Roadside parking
 Bus stops
Factors that affect intersection delay (2/3)

Traffic conditions
 Peak-hour
operations of traffic
 Turning movements
 Traffic composition
 Approaching speed
 Pedestrian
 bicycle
Factors that affect intersection delay (3/3)

Intersection control conditions
 Intersection
type
 Traffic signal control
 Signal cycle length
 Phase split
 STOP or YIELD control
 Protected or permitted left/right turns
Representation of Delay by Vehicle
Trajectories
Segment Delay
Distance
Control Delay
T0
Stop Line
Queue Delay
T2
T1
Time
Target Speed
Stopped Delay
Acceleration
Deceleration
Running
Speed
HCM 2010, Exhibit 7-8
Definition of delay terms
Time-in-queue Delay
Time-in-queue Delay: Total time from joining a
queue to passing the stop line
Control delay


Control delay is the additional travel time caused
by operation of a traffic control device.
This delay definition is the one used by the
procedure of the United States Highway Capacity
Manual (US HCM) for assessing Level of Service
(LOS) at controlled intersections and roundabouts.
Segment delay


Segment delay is more commonly used by
simulation tools. It reflects the delay experienced by
each vehicle since it left the upstream node (usually
another signal).
Segment delay includes control delay plus all other
delay due to traffic interactions.
Stopped delay



Stopped delay reflects the amount of time a vehicle
was actually stopped.
Since a vehicle is considered to be stopped if it is
traveling at less than a threshold speed. The beginning
and end of a stop are generally based on speed
thresholds, which may differ among tools. In some cases,
the threshold speeds are user definable.
Earlier version of the US HCM defined stopped delay
as 76% of the control delay, on the basis of empirical
data.
Queue delay


Queue delay reflects the amount of time a vehicle
spends in a queued state.
A vehicle is considered as having joined the queue
when it approaches within one car length of a
stopped vehicle and is itself about to stop. This
definition is used because of the difficulty of keeping
precise track of the moment when a vehicle comes to
a stop.
Aggregated delay verse unit delay


It is important to note the difference between
aggregated delay, usually expressed in vehicle
hours, and unit delay, usually expressed in seconds
per vehicle.
For these two definitions to be dimensionally
consistent, the unit delays must actually be
expressed in vehicle seconds per vehicle. It is
common practice, however, to shorten the definition
to seconds per vehicle to promote public
understanding.
Aggregated delay verse unit delay


Aggregated delay is generally used to assess the
operating costs associated with a candidate
treatment, because an economic value can be
assigned to a vehicle hour of delay.
Unit delays are associated with driver perception of
the LOS on a facility.
Introduction of this section




Much has been written, using Highway Capacity
Manual methodology, regarding the computation of
capacity and levels of service at intersections.
But often overlooked is the fact that field observations
are the basis for the computations.
The amount of delay encountered by the typical vehicle
using an intersection approach determines the Level of
Service for that approach.
Direct measurement of this delay provides a quick,
reliable method for determining the effectiveness of an
intersection.
Introduction of this section(cont.)



This section focuses on field and office procedures that
allow the user to measure the average vehicle delay
per approach and thereby assess the performance of
an operating signal.
This procedures illustrate how the stopped delay can be
measured to approximate the control delay now used
by US HCM to determine Level of Service.
Field assessment of average delay per vehicle, and the
translation of that observation into HCM-designated
levels of service, is a powerful tool for many traffic
engineering applications.
Measurement of intersection stopping delay




Invented by The university of California at Berkeley
in 1954.
Simple
No special equipment is required
Have being widely used since then
Equipment




No special equipment is needed to complete the
procedure.
Proper note-taking materials, including field data forms,
and a stopwatch or wristwatch with a second hand are
all that are required.
Other items that can be helpful are a laptop computer
or calculator programmed to aid in data entry and a
video camera.
If properly placed, the video camera will allow data to
be collected simultaneously on multiple approaches and
for multiple movements on a single approach.
Data Collection
Visit the site to determine the best observation
points and time for observations
 Collect and record the data
 Check your work before leaving the field

Visit the site to determine the best observation
points and time for observations
As in all data-gathering efforts, the observer
should ascertain the best time of day to collect
the data.
 Usually, a delay study is associated with the
peak hour for the intersection, for a particular
approach to an intersection, or for a nearby
major traffic generator.

Determine which lanes or lane groups
should be measured
Before collecting the data, the analyst must
decide which lanes or lane groups should be
measured.
 The HCM recognizes lane groups as the group
of lanes on an approach from which drivers
can complete the same move during the same
signal phase.

Concept of lane groups


For example, a two-lane approach to an
intersection that has a dedicated left-turn lane and
a combined through and right-turn lane would have
the delay determined separately for each lane.
However, a two lane approach composed of a
combined left-turn and through lane and a
combined right-turn and through lane – all
controlled by a single phase – is considered to be a
lane group and would warrant a single delay
determination for the entire lane group.
Data Collection





The procedure for collection of the data is quite
straightforward.
Using a fifteen-second interval, the number of
vehicles stopped on the lane group is recorded.
Vehicles and motorcycles are recorded separately.
Data recording for this purpose assumes that each
of the stopped vehicles has been stopped or
delayed for fifteen seconds.
Some vehicles will be counted more than once if
they are stopped for more than on interval. That is
acceptable.
Data Collection


Simultaneously, using a one-minute recording
interval, the number of vehicles using the lane group
on the approach – and whether they must stop or
not – is also recorded.
Assuming that all vehicles which stop is
approximately fifty percent, the procedure should
continue until a minimum of 350 vehicles are
observed entering the intersection via this lane
group.
Minimum sample sizes
Allowable Errors
d
Significance of Confidence
90%
95%
99%
5%
1,084
1,536
2,652
10%
271
384
663
Source: 交通調查與分析(第二版)，王建軍、嚴寶杰編著，人民交通出版社，2001年
Data Collection

It may be helpful to have four observers per lane or
lane group, with
 One
timing and alarming the fifteen-second marks
 One collecting and recording the number of stopped
vehicle at each of the fifteen-second marks
 One collecting the total number of vehicles entering the
intersection – in one-minute recording interval
 One collecting the number of vehicle entering the
intersection without stopping – in one-minute recording
interval
Check our work before leaving the
field



The columns should be quickly summed and the average
delay computed before leaving the field.
These computations will be checked later during the
analysis phase but are computed here in order to check
that the collected data is reasonable.
It should be noted that since one vehicle may stop for
more than one observation periods, e.g. 15 seconds,
and should be counted as a stopped vehicle in the
consecutive 15-second intervals. But that vehicle was
only counted once while recording the approach volume
of vehicle stopped. Thus, it is a good checking point that
for each recording time period, one minute in this case,
the approach volume of vehicle stopped should be less
than or equal to the total number of vehicle stopped.
Data analysis
Summarize field data
 Calculate total and average control delay
 Compare calculated delay to Highway
Capacity Manual level of service limits
 Summarize and draw conclusions

Summarize field data




To summarize the data, add each of the columns as was
done on the example data collection form.
This should be done as a preliminary in the field, if
possible, so that any anomalies can be revealed.
Be aware that the summation of vehicle delays and the
total volume may not coincide, since they are two
entirely different data items.
The delay data may reflect vehicles that were delayed
more than one interval, thus counted more than once,
whereas the volume numbers reflect each vehicle
counted once and only once.
Calculate the total delay



The total delay observed is calculated. This value is
simply the number of delay intervals multiplied by
fifteen seconds.
For the example, this is 154x15=2310 vehicle-seconds
of delay.
One vehicle-second of delay is one vehicle delayed
for one second.
Calculate the average delay per
stopped vehicle




The total delay divided by the total number of
vehicles delayed equals the average delay per
stopped vehicle.
For the example data this is 2310/119=19.4
seconds per stopped vehicle.
This value indicates how long a vehicle is delayed if
it is required to stop on the approach.
It is not used to determine the Level of Service for
the approach.
Calculate the average delay per
vehicle entering the intersection



The average delay per vehicle is computed by
dividing the total delay by the total volume.
The US HCM procedure multiplies this value by 0.9
to compensate for sampling error.
A correction factor is added to account for the
effect of the average number of vehicles
proceeded per cycle. And, adjustments are made
for acceleration and deceleration time.
Calculate the approximation of control
delay


A very close approximation of the values obtained
by the US HCM procedure is calculated by dividing
the total stopped delay by the total volume and
multiplying the result by 1.3.
For the example data, this is
(2310/295)(1.3)=10.2 seconds per vehicle. This
value is a close approximation of control delay and
can be used to identify the Level of Service
encountered by the traffic.
Calculate the percentage of total
volume delayed


The percentage of total volume delayed is
computed to provide insight into what portion of the
traffic is forced to stop at the intersection.
For the example data, this is 119/295=0.403 or
40.3%.
Compare calculated delays to Level of
Service limits

According to the US HCM, the Level of Service is
determined by the average control delay per
vehicle, using the US HCM limits for each Level of
Service.
US HCM Levels of Service
Level of Service
Control delay per vehicle, sec
A
≤ 10.0
B
10.1 to 20.0
C
20.1 to 35.0
D
35.1 to 55.0
E
55.1 to 80.0
F
> 80.0
Source: United States Highway Capacity Manual
Summarize the results



If all of the intersection’s lane groups have been
studied, then the results should be presented
graphically to facilitate an understanding of their
interaction.
This type of presentation will immediately identify
those portions of the intersection that are suffering,
and it will allow the traffic engineer to determine if
the signal is performing as intended.
If desired, a weighted average of the average
delay per vehicle can be computed to identify an
overall Level of Service for the intersection.
Draw conclusions

Several questions should be addressed with this
findings.
 How
does this intersection’s performance compare to
rest of the system’s performance?
 Can things improve at this location?
 Should there be an ongoing delay study of this location
as part of a “preventive maintenance” program?

The question of capacity of the intersection is
omitted from the previous list of questions. Capacity
can be determined by measuring saturation flow
rates, as explained elsewhere in this class.