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10. Traffic Control Devices

10.1 Introduction

Traffic control devices can be defined as all the signs, traffic signals, pavement markings, traffic islands, or other devices placed or erected to regulate, warn or guide traffic.

These devices will only be effective if placed or erected by a traffic authority having the necessary jurisdiction to enforce regulations pertaining to the devices e.g. a stop sign will be of little use unless someone has authority and jurisdiction to prosecute those who do not obey the sign.

Driver expectations must always be accounted for when installing devices and this means that as far as practicable only standard devices should be used. Devices will only command respect and attention from drivers if they can be readily seen and read, and if their messages appear reasonable under the circumstances.

10.2 Standards, Guides and Warrants

Standards for signs, markings, etc. in regard to designs, sizes and usages can be found in the following documents:

The Australian Standard Manual of Uniform Traffic Devices – A.S. 1742. Part 1 contains description of the elemental devices which have been adopted as an Australian Standard and provides the basis for signing and marking practices throughout Australia. Part 2 contains standard applications of devices to traffic situations.
The United Nations Convention on Road Signs and Symbols. This document is notable for its wide use of symbols on regulatory and warning signs, and as Australia is a signatory to the Convention, progressive inclusion of most of its provisions in AS 1742 and future amendments has already commenced and can be expected to continue.
Australian Standards AS1743 and 1744 which contain manufacturing standards for signs and details of standard alphabets.
Manual of Uniform Traffic Control Devices. This document has legal significance as it is the document referred to in the Traffic Act, Queensland.

10.3 Signs

Australian road signs may be classified into four types:

Regulatory Signs.

These signs are generally rectangular with the long axis vertical, and have a black legend on a white background. Special shapes and colours are specified for these signs where there is a need for easy identification e.g. Stop and Give Way signs. Regulatory signs inform road users of traffic laws or regulations which it is an offence to disregard.

Warning signs.

Warning signs are generally square with one diagonal axis vertical, and having a black legend or symbol on a yellow background. They are used to warn traffic of hazardous or potentially hazardous conditions in or adjacent to the road. Warning signs advise the driver of conditions which require caution, and possibly a reduction in speed.

Guide Signs.

These signs are generally rectangular with the longer axis horizontal, and the usual colour combination is a white legend on a green background. However several exceptions occur, including route markers, street name signs, tourist signs and signs indicating roadside facilities. Guide signs inform and advise road users about the direction and distances to destinations in the road system, inform them of services or features of interest along the road, and give instructions which although not regulatory should be obeyed.

Signs for Roadworks and Special Purposes.

These signs are used at road works to warn road users of temporary hazardous conditions which, if attention is not directed to them, would either create a danger to traffic or place the men engaged on the road works at risk from passing traffic. Typical examples are road and bridge works, temporary road closures caused by flood, landslides or other hazards, or a temporary re-routing of traffic. These signs are rectangular with black letters and numerals on a reflectorised yellow background, but certain signs used to protect workmen during daylight hours only have a fluorescent yellow-orange background.

LINK TO THE PICTURE GALLERY

The Picture Gallery contains pictures of:

10.4 Pavement Markings

Pavement markings are all the lines, symbols, patterns, messages, numerals, or other devices set in the pavement or applied or attached to the pavement or kerb to regulate, warn, or guide traffic. They may act as a supplement to other road devices but often they are the only effective way to convey certain regulations and warnings to drivers.

Pavement markings have the following limitations:

they may not be clearly visible if the road is wet or dusty;
they are subject to traffic wear and usually require frequent maintenance;
they may be obscured by traffic;
their effect on skid resistance requires a careful choice of materials; and
they cannot be used on unsealed roads.

Despite these limitations their advantage under favourable conditions is that they convey information to the driver without diverting attention from the roadway. A strong contrast between the pavement and the pavement markings is a primary requirement. White is used for most Australian markings, except that yellow may be used as an alternative for barrier lines and for parking lines which indicate restricted parking areas.

Raised pavement markers may be used to supplement and, in some instances, replace painted lines on the road surface. They may be reflective or non-reflective and in some circumstances a combination of both types may be appropriate.

The following types of pavement markings are used.

Longitudinal Lines:

Separation lines (Centre lines)
Lane lines;
Edge lines; and
Barrier lines.

Transverse Lines:

Stop lines;
Holding lines; and
Pedestrian crossing markings.

Other Markings:

Turn lines;
Diagonal and chevron markings;
Pavement messages;
Parking and loading area markings; and
Kerb markings.

10.5 Guide Posts and Guard Fencing

Guide posts, with or without reflectorised delineators (commonly known as reflectors), are placed in series on one or both sides of a road formation to indicate to road users the alignment of the roadway ahead, especially at horizontal and vertical curves. The delineators act as effective aids for night driving. Red delineators should be used on the left side of the road and white delineators on the right.

The normal section for guideposts is currently 100 × 50 mm with the wider width facing oncoming traffic. Posts are generally coloured white. Posts are generally made from sawn timber, although other materials may be used provided the posts do not constitute a hazard to vehicles.

Guard fencing is of particular value in delineating the roadway, in deflecting traffic from hazardous locations, and in reducing the severity of accidents where vehicles may otherwise leave the road formation. Corrugated guard railing is now commonly used, although the older types of post and cable, post and chain wire, and wooden post and rail can still be found.

The use of guard fencing is recommended in the following circumstances:

sub-standard curves;
road embankments (greater than 5 m in height and with side slopes greater than 1 in 4);
narrowing of formation;
on the outside of curves on steep down grades;
protection at structures (e.g. bridge piers);
protection for pedestrians; and
median separators.

10.6 Traffic Islands

A traffic island is a defined area within a roadway from which vehicular traffic is intended to be excluded, and which is used for the control of vehicular movements or for pedestrian refuge. The term island as used in this section includes medians and separators.

Traffic islands are designed to perform one or more of the following functions:

direct vehicular traffic to minimise traffic conflicts;
separate traffic streams;
separate points of conflict;
prevent or redirect undesirable traffic movement and turns;
improve the efficiency of operation at intersections;
provide protection for pedestrians; and
provide suitable positions to locate other traffic control devices.

10.7 Traffic Control Signals

Traffic signals are an increasingly popular method of intersection control since they can usually provide a quick and relatively cheap (compared to one-way street systems or grade separations) remedy to intersection problems.

The basic principle of traffic signal design involves the allocation of ‘green-time’ to vehicle movements in proportion to the vehicle volumes entering and the available approach saturation flows for the intersection.

10.7.1 Types of Traffic Signal Control

Pretimed Signals

With these signals green signals appear on each approach of a signalised intersection and the time that signal remains green may be predetermined and fixed in the signals equipment by a timing device. Signals which operate this way are called pretimed signals, or fixed time signals. Pretimed signals cannot take account of fluctuations in traffic flow and are not, therefore, highly efficient but they are relatively cheap to install and maintain. If fluctuations in traffic flow are small they are satisfactory. The efficiency of fixed-time signals can be improved by providing a number of different cycle lengths, usually three, to control morning peak, off- peak, and afternoon peak traffic.

Vehicle-actuated Signals

With this system vehicles on any intersection approach are sensed by some detecting device, e.g. magnetic loops or pneumatic pads placed in the roadway. By recording the vehicles as they cross the detector and by timing the intervals between vehicles the signals are automatically adjusted to give preference to the approach with the heaviest flow. Vehicle-actuated signals, as distinct from pretimed signals, can take account of fluctuations in traffic flow so that in light traffic conditions delays are less.

Pedestrian-actuated Signals

This type of signal should only be provided at mid-block locations to temporarily interrupt the flow to permit pedestrians to cross the road.

10.7.2 Co-ordination of Traffic Signals

An urban arterial which has a series of signalised intersections may be co-ordinated for efficient movement of traffic. In business districts it may be desirable to establish a comprehensive system which covers the entire area and provides co-ordinated movement in all directions.

More recently attention has been given to using computers as a means of optimising traffic flow which is controlled by traffic signals. Where there are a significant number of signals over a wide area, and co-ordination for several major road arteries is needed, on-line computers can economically and functionally meet such a need.

LINKS TO SITES ON TRAFFIC ENGINEERING

The Arizona Department of Transportation has a site "Traffic Engineering: What Why How " which contains a useful overview of what traffic engineering is about, what traffic control devices are, how speed limits are determined, and other interesting issues related to traffic engineering.

Page last modified 9 June 2005.