Urban Transport Strategy. Management in Developing Countries John A Cracknell, страница 112

Lessons - Many traffic systems suffer from inadequate signing, poor siting of signs, lack of visibility of signs (especially at night), inadequate signing standards, poor materials and lack of budgets for continuous signing and marking upgrading and maintenance.  Regulatory and warning signing and road marking are essential elements of any traffic regime.  It is indisputable that good, city-wide signing and road marking can assist development and maintenance of driver discipline, safety and can support enforcement.  In general, as a result of the disappearance of direct labour agencies in cities, the Bank has no project investment participation in signing and road marking programs.  Improvements in general marking and signing are closely linked to the existence of an effective traffic management agency – even if all works are contracted, the planning and supervision of signing and marking measures should be an important function of the city traffic agency (see Chapter 4 – Institutional Arrangements)).

Traffic Capacity Improvement at Junctions

Rationale and Objectives – urban road networks tend to operate below potential capacity, largely attributable to the constraints imposed by junctions.  Junctions are the critical capacity elements in an urban road system and are where delays, accidents and emissions are concentrated.  The objectives of a junction improvement program are to:

§  alleviate traffic congestion by providing adequate capacity for vehicular traffic such that the level of service at the junction is compatible with that provided on the upstream/downstream roads

§  ensure that the capacity provided is consistent with the traffic management strategy of favouring “people not vehicles”; where buses-bus passengers are important elements of the junction flow, economic evaluation should be used to determine the “optimum capacity” arrangements;

§  minimise accident risk and severity, particularly for vulnerable users such as cyclists and pedestrians

§  minimise adverse environmental impacts of traffic by reducing fuel consumption through reductions in the number of speed changes and the number of stops/starts required at the junction;

Types of Intervention – The hierarchy of alternative junction layouts which cater for increasing levels of traffic flows are:

§  uncontrolled junctions without any designated priority;

§  priority junctions;

§  roundabouts

§  isolated traffic signal controlled junctions

§  junctions under ATC

§  grade–separation

Inclusion in Bank traffic management projects/component – all traffic components in Bank projects invariably include junction improvements.  It has been noted in Chapter 1 that the current Paper is not intended as a design manual it would be unrealistic to present design guidelines – innumerable manuals exist (ii) nor would it be possible to summarise world-wide experience of junction improvements – design depends on unique city circumstances.  Thus, this section of the Paper presents some qualitative observations of common junction operation and planning issues in developing cities.

Issues and Lessons of Junction Design – all junctions should be planned and designed in accordance with a range of design reference traffic flows and to recognise site constraints.  Some particular issues and lessons which apply in developing cities include:

§  Traffic signal timings at existing junctions are often not adjusted to deal with actual traffic distribution. Traffic flows and distribution change over time due to growth in vehicles, changes in development etc and many traffic signals at isolated junctions are not adjusted to deal with these changes.  Appropriate signal timing settings can increase junction capacity at virtually no cost but the simple action requires (and therefore reinforces the need for) a competent traffic management agency to monitor signal timings and to make the necessary modifications on a continuous basis;