9. Traffic Studies and Parking
9.1 Purposes of Traffic Studies
A traffic study is the collection and analysis of
measurable factual data relating to traffic and its characteristics.
Traffic studies are carried out to:
- provide a basis for planning and designing traffic
facilities, including the selection of geometric standards, economic analysis,
and the determination of priorities;
- assist traffic operation by determining the need
for traffic control devices such as signs, traffic control signals, pavement
markings, and school and pedestrian crossings;
- evaluate the effect of changes made for traffic
by conducting before and after studies; and
- determine the basic characteristics and the general
laws of traffic behaviour.
9.2 Traffic Volume Counts
Traffic volume counts are made to determine the number
of vehicles passing a point. These counts may be comprehensive counts covering
the entire main road system in an area, counts on all roads intersecting
a cordon line which encircles a particular area, counts on screen line(s)
which divide a city into two or more parts, or counts at specific points.
The information sought may include traffic volume and the direction of traffic,
volume of turning traffic at intersections, hourly, daily, and seasonal variations
of traffic, or proportion of cars, trucks and buses.
Traffic volume counts can be carried out either
manually or by automatic traffic counters. Manual counts are usually undertaken
at points where it is necessary to record the proportion of vehicles as well
as the traffic volume, at intersections where the volume of the various turning
movements is required, or at sites being investigated for installation of
pedestrian crossings and guard-controlled crossings.
Equipment used in automatic counts normally consists
of a device to detect the passage of axles or vehicles, and a metering device
to record the number of axles or vehicles detected. The detecting device most
commonly used in Australia is the pneumatic detector. A rubber tube is laid
across the roadway and as a vehicle’s wheels pass over it an air impulse
is sent along the tube to the meter. Two main types of counter are used to
measure traffic volumes, the non-recording counter and the recording counter.
The non-recording counter accumulates the number of vehicles detected and
must be read at regular intervals over the time it is desired to obtain traffic
counts, e.g. if 24-hour counts are required the counter must be read every
24 hours. The recording counter records the traffic volume passing each hour
(in some cases each quarter hour, or each five minutes) by printing numbers
on, or by punching holes in, a moving paper tape, by recording on magnetic
tape, or by recording in a microprocessor which is later downloaded to a
computer.
The method of presenting results will depend on
the purpose of the study. In comprehensive area traffic counts the Average
Annual Daily Traffic (AADT) volumes are usually presented as traffic flow
maps in which flow bands proportional to the AADT volumes are drawn along
the route surveyed.
9.3 Origin and Destination Surveys
Traffic counts give the amount of traffic passing
specified points on the road but they do not indicate where traffic desires
to travel, i.e. its origin and its destination. An origin and destination
survey is designed to obtain such information about the movement of vehicles
and passengers within an area. The survey is primarily for transportation
planning, particularly the location, design, and programming of new or improved
highways, public transport, and parking facilities. An origin and destination
survey may range from a relatively simple study to determine the amount of
traffic that would by-pass a town to a comprehensive transportation survey
for planning and design of the transportation system in a large metropolitan
area.
Methods used for Origin and Destination surveys
include:
- Recording registration numbers;
- Handing postcards to drivers;
- Roadside interviews;
- Tag-on-vehicle surveys;
- Headlight surveys; and
- Home interview surveys.
9.4 Speed Studies
Vehicle speeds can be measured in the following ways:
- using a radar meter, which gives a direct reading
of speed for each vehicle;
- taking photographs of a section of road at a
predetermined time interval t, measuring the distance x covered by each vehicle,
and computing speed from: v = x/t
- timing successive vehicles, or a representative
selection of vehicles, over a short predetermined distance of length a.
Results may be presented in tables, graphs and diagrams.
These may include speed distribution and cumulative frequency distribution
curves. The more important quantities are the mean speed and the 85th percentile
speed.
9.5 Travel Time and Delay Studies
A modified form of the speed study is the travel
time and delay study. It measures the average journey time and journey speed
of sections of a route (or routes) and is used in traffic assignment, to
assess the quality of the traffic route, or to evaluate the before and after
effect of traffic engineering techniques or other changes made for traffic.
By analysing the delays, the location and cause of the congestion can be
identified and remedied.
9.6 Crash Studies
The establishment of an accurate and efficient crash
records system is of prime importance to the traffic engineer. Crash
records are used by the traffic engineer:
- to identify hazardous locations and the causes
of crashes at them;
- as an aid in determining priorities for road
improvements or the installation of traffic control devices;
- to evaluate safety improvements by before and
after studies;
- as a measure of the level of service provided;
and
- to carry out fundamental research into the relationship
between traffic crashes and the road environment.
Sources of data on crashes are reports of individual
crashes, and the information recorded varies from State to State.
Crash data may be conveniently represented by:
- Crash Spot Maps
An crash spot map shows the location of each
crash and the type of crash by coloured pins, e.g. fatal crashes
by red pins, injury crashes by blue pins, etc. The maps which are large
scale and attached to display boards are kept up to date as crash report
forms are received. The maps provide a quick visual guide of where crash
concentrations exist.
- Collision Diagrams
A collision diagram illustrated pictorially, by
means of directional arrows and symbols, the path and nature of collision
of vehicles and pedestrians involved in crashes at a particular intersection
or stretch of road. The diagrams are not usually drawn to scale, and distances
between high frequency locations may be shortened. The arrows do not show
exact paths as they would overlap and become confusing. Accordingly, crashes
of the same type are grouped together. A study of the crash pattern portrayed
by a collision diagram may suggest possible improvements such as installation
of traffic signals, channelisation, removal of obstructions etc. which may
eliminate a certain type of collision.
- Crash Summary Forms
The use of a standard crash summary form is
recommended when routine crash histories are required. The form contains
the same type of information for each crash occurring at a requested
location between specific dates. Although not having the visual impact of
a collision diagram a crash summary form provides more detail.
9.7 Parking Studies
Parking studies are carried out to indicate:
- the number and location of existing parking spaces,
both kerbside and off-street;
- existing parking practices, including usage of
available spaces, parking duration, illegal parking;
- the need to impose or vary parking time limits
or to install parking meters; and
- the adequacy of existing enforcement measures.
A suitable method for carrying out a parking survey
in a small city is as follows:
- Select the study area. This should include
the portions of the town or city containing most of the business activity
and should also include those areas that generally cater for parking.
- Carry out a parking inventory in the study
area. This involves the collection or preparation of suitable base maps of
the study area showing all streets, lanes, etc., the location and type of
all buildings and vacant areas, all kerb space available for legal parking
(this is obtained from a detailed survey), areas where vehicles cannot legally
park (e.g. close to intersections, school or pedestrian crossings, etc.),
any off-street parking lots, garages, or service stations with details of
available spaces, fees, etc.
- Carry out a cordon count. All vehicles entering
and leaving the study area should be counted at 15 or 30 minute intervals
for the period from say 7.00 am to 7.00 pm. Determine the number of vehicles
accumulated in the area at any particular time and the time of day when
it occurs. In small towns cordon counts may be omitted.
- Carry out a parking usage study. This study
should extend from say 8.00 am to 6.00 pm, the active hours of the day. Observers
should tour each kerb and parking lot or garage every 15 or 30 minutes and
record the registration numbers and type of vehicles at each parking space.
Illegally parked vehicles should also be recorded. Close to certain generators
such as a post office, it may be necessary to make checks at shorter time
intervals (particularly if 30 minutes has been used as the general interval).
The area can be divided so that the survey can be spread over several days
which have a similar parking demand. At off-street parking stations the actual
distribution of the parking usage can be obtained from the tickets.
For larger cities, a comprehensive parking demand
study may be required, which includes the determination of parking usage,
parking habits as well as the origin, destination, and purpose of trip of
drivers parking in the area. It is used primarily in determining the demand
for parking space by evaluating the individual parker’s desires. The actual
survey is carried out in the form of questionnaire cards or direct interviews.
LINKS TO SITES ON TRAFFIC STUDIES.
The Australian Department of Infrastructure, Transport,
Regional Development and Local Government's website contains national data and
associated reports concerned with road traffic fatalities at "Road
crash statistics".
9.8 Types of Parking Facilities
The provision of adequate parking facilities is an
essential part of the design and construction of transport infrastructure.
Provision for parking may conveniently be thought
of in three categories:
- on road parking (kerbside or centre-of-the-road);
- off road single level; and
- off road multi-level.
Whichever type of facility is to be provided it will
be necessary to design for a specific size of vehicle, with certain manoeuvring
characteristics. A ‘design vehicle’ must therefore be established as part
of the design process.
Geometric design standards for parking facilities
should take into account not only the operational disadvantages of under-design
but also the financial disadvantages of over-design. In some sections of
parking design standards lower than the ideal can be tolerated. Sections
of parking facilities can be divided into two categories – critical areas
and non-critical areas. Non-critical areas will usually be designed to satisfy
about 85% of common vehicles and hence the 85 percentile design vehicle is
used. Critical areas must be designed to cater for all vehicles (or, economically,
nearly all vehicles) so that a 99 percentile design vehicle is used.
9.9 Design Vehicles
The recommended minimum dimensions for design purposes
are the dimension just allowing movement by the appropriate design vehicle
plus a clearance around the vehicle. Austroads in its publication "Guide
to Traffic Engineering Practice" gives the following guide dimensions of
a design vehicle for on-street parking:
| Description |
Dimension (m)
85 percentile |
Dimension (m)
99 percentile |
| Overall length |
4.74 |
5.37 |
| Overall width |
1.86 |
1.89 |
| Front overhang |
0.813 |
0.996 |
| Rear overhang |
1.100 |
1.300 |
Guide Dimensions for a Design Vehicle for
Kerbside Parking
9.10 Kerbside Parking
The major purpose of a road is to provide for the
safe and free movement of road traffic. Another function of many roads is
to allow vehicles to park at the kerbside to allow the access of people to
adjacent properties. However, the act of parking or unparking can interfere
with the free flow of vehicles in the adjacent lane. In addition parked vehicles
contribute to pedestrian accidents with persons entering the roadway between
or behind parked vehicles. The use of kerbside space on streets to accommodate
stationary vehicles therefore should be regulated to provide the optimum
conditions for all road users considering safety for motorists and pedestrians,
vehicle flow, and the need for on-street parking to serve adjacent properties.
Angle parking is more convenient than parallel parking
but it invariably produces a much higher accident rate than parallel parking
at the same location. Angle parking is not adaptable to commercial vehicles
and requires substantially larger street widths. The angle parking manoeuvre
is easier to accomplish and causes less interference to traffic than the parallel
parking manoeuvre, but the unparking manoeuvre is not as easily executed.
Other disadvantages are that motorists drive more slowly when looking for
vacant spaces and the car’s overhang at the kerb is greater interfering with
pedestrians, awning posts, and signs on the footway. Angle parking accommodates
more cars for a given length of kerb than parallel parking.
The decision on the type of parking allowed in any
street must be based on:
- the width of road;
- the volume and type of traffic;
- the turnover expected;
- the nature of neighbourhood; and
- the road classification.
Taxi stands should be distributed at convenient locations
for patrons throughout business centres. Stands should normally be restricted
to accommodate no more than four taxis at a time. If the demand is greater
the main taxi stand should be fed from feeder stands established nearby.
The length of a taxi stand should be (5.5n + 1.5) metres where n is the number
of taxis to be accommodated.
Although some establishments provide off-street
loading and unloading bays, most loading and unloading takes place at the
kerb and provision for loading zones must be made. The length of loading
zones can vary greatly depending on the number and type of establishments
served by each zone and the usual type of vehicles using the zone. The recommended
minimum length is 9 m.
9.11 Centre-of-the-Road Parking
This type of parking can only be considered for very
wide streets and should be avoided unless adequate provision is made to
separate through traffic from parked vehicles, motorists looking for spaces,
and vehicles parking or unparking. One disadvantage is that pedestrians leaving
and returning to their vehicles have to cross the main traffic stream to
reach the footway.
Where vehicles have direct access to parking spaces
from main traffic lanes they may cause unnecessary interference with through
traffic, reduce the traffic flow, and produce a greater proportion of accidents.
9.12 Off-Street Parking
Off-street parking requirements normally depend on
the extent and type of development in the area, and the availability of
on-street parking spaces.
The design problem for off-street parking areas
is basically one of layout of parking spaces, both to maximise the number
of parking spaces and also to facilitate circulation and access to those
spaces. Layouts usually involve either angle or perpendicular parking. Parallel
parking is rarely used, except perhaps at the boundaries of the area where
insufficient space may remain to allow any alternative arrangement.
The majority of car parks in Australia are constructed
with 90° parking because it produces the most efficient layout in most
cases.
Design for efficient circulation of vehicles within a parking lot should
aim at ease of movement around the site and safety for both vehicles and
pedestrians. A definite circulation plan should be developed, taking into
account the different activities which occur at the site; for example, the
design for a supermarket parking area should allow for pedestrian movements,
parcel pickup, deliveries, etc.
Site entry and exit is always an important component of the design. The
location of entry and exit points depends not only on conditions and activities
within the site, but perhaps even more importantly on interactions with traffic
on the road or roads adjacent to the site. In some instances it will be more
efficient to combine entry and exit at the one location, while in other cases
internal circulation and external traffic considerations will indicate that
separate entrances and exits should be provided. Allowance for adequate queue
storage space is another issue associated with entry and exit, especially
the avoidance of queues extending into the street at entrances.
Special problems may have to be considered in the design of parking areas
for public facilities such as sports arenas and concert halls, which have
very large demands concentrated over short periods. Circulation and entry/exit
design must aim for maximum efficiency to enable rapid evacuation of the parking
area as the conclusion of the event attended by the parkers.
9.13 Multi-Level Carparks
Multi-level car parks concentrate a high number of
parking spaces into a small area, and also have the potential to concentrate
parking problems within the area. However, in some respects they are simpler
to design than single level car parks because heavy vehicles and buses are
usually excluded from their operation. Also the circulation system for vehicles
is often simpler. Multi-level car parks are normally only proposed in areas
of high density development where it is economical to construct a multistorey
structure rather than to acquire land for single level parking.
The special problems associated with multi-level
car parks can be summarised as follows:
- Presence of columns in the structure, which restricts
access to parking spaces and which may inhibit door opening of vehicles.
- The geometry and grades of ramps between parking
levels.
- The concentration of queuing and circulation
problems at the level or levels at which entry and exit take place.
- The often long driving paths to a vacant space
encouraging operating speeds higher than those desirable.
- Provision of ticketing and pay facilities.
9.14 Operation of Off-Street Parking Facilities
Many off-street parking facilities are open to the
public free of charge and provide for each driver to park his or her own
vehicle. These are usually surface lots and may be provided by municipalities,
retail complexes, etc. Such facilities require almost no administration or
control once they are established, apart from periodical maintenance and,
in some municipal car parks, the same enforcement of parking duration limits
as is necessary for on-street parking.
A significant proportion of off-street parking facilities,
however, do not fall into the above category and their method of operation
depends upon whether self-parking or attendant parking is to be used and involves
the administration of parking fees.
LINK TO THE PICTURE GALLERY
The Picture Gallery contains photographs relating
to Carparks
.
LINKS TO SITES ON PARKING
The International
Parking Institute
in the USA has some useful and interesting information at their Web site.
The Frequently Asked Questions section (within the Resources Center) mentions
that there are an estimated 105 million parking spaces in the USA, that there
is estimated to be 5 million parking meters worldwide, and that the New York
City gains revenue of $300 million per annum from parking related fines!
Brisbane City have information on
Carparking
within the City. Some people that park in Brisbane are not aware that in
the Central Traffic Area (the Central Business District and surrounding
areas) the parking time limit is 2 hours, even if the parking space is not
designated with a time limit.
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Page last modified 28 June 2010.