7.1 Traffic measurements & surveys
Vehicles are usually measured in
traffic terms as passenger car units (PCUs). A car is one PCU,
a bus is 2 or 3 PCUs. 10 cycles equals 1 PCU. These measurements
are used in traffic flow and signal calculations. That is X PCUs
per minute or hour. Traffic volumes vary widely between busy times
in the morning and evening “peak” times, and during
the night. They also vary between days of the week and seasons
of the year. Poor weather and darkness also affect traffic flow.
Therefore data on traffic volumes needs to state clearly if it
relates to average or maximum flows and the time scale.
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7.2
Capacity & safety:
Junction
design with PICARDY
Junctions are where vehicles turn
and across the path of other vehicles and pedestrians want
to cross the road. These conflicts are dangerous and cause
accidents. If speed and volumes are sufficiently low, drivers
and pedestrians have time and space to avoid collisions.
But in many cases speeds and volumes are high. At a typical “T” junctions,
vehicles entering from the side onto a busy road have to wait for
gaps in the traffic. Computer programs such as PICADY help calculate
how to allow drivers to turn right, for example, safely and at
what stage a roundabout or traffic signals would be needed.
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7.3
Roundabouts: ARCADY
Full sized roundabouts are designed to cope with large volumes
of traffic making complicated turns. However they take up more
room that traffic signals and are difficult for pedestrian to
cross. New designs for “compact” roundabouts take
up less space and require drivers to enter the roundabout more
slowly. This should reduce accidents and make it easier for pedestrians
and cyclists.
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7.4
Mini & informal roundabouts
Intended for relatively
low speed areas, mini-roundabouts have a prescribed set of road
markings and signs, as well as rules about not passing over the
central roundel. A recent innovation, an informal roundabout, has
no formal signs and lines, merely a change of road surface texture.
Drivers appear to cope well and there are considerable visual advantages.
Street clutter is reduced to a minimum.
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7.5
Signals
In their simplest form,
traffic signals stop vehicles in a lane of traffic so that those
from other lanes can cross or turn safely in front of them. They
also allow pedestrians to cross safely. The signals are organised
to allow vehicles to move forward in a series of “stages” so
that each lane has its turn. The signal timings are calculated
to within seconds so that road space is used efficiently. This
sometimes means that pedestrians are required to make considerable
detours from their preferred route and be delayed in central refuges.
Unfortunately the comfort and convenience of pedestrians is not
assumed to be a high priority in signal computer programs.
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7.6 Traffic signs: Reducing street clutter
There are three purposes
for traffic signs: to instruct, warn and inform. Signs to instruct
(regulatory signs) are mostly to enforce a traffic regulation order.
They are usually essential for the safe movement of traffic. Warning
and information signs are less essential and they can often be removed
or relocated. But even regulatory signs can be reduced to a minimum
and the Traffic Signs Regulations and General Directions, which describes
and illustrates them, spells out ways this can be done.
No entry sign (sign 616) need only be put at one side of the
road where the sign is at a junction and the road is less than
5.0 metres wide. At roundabouts, turn left signs (sign 606) is
not a legal requirement. The chevrons (sign 515) can also be
omitted. The test to apply is to consider if there is likely
to be any doubt that a driver is approaching a roundabout. Car
parking signs (sign 661 series) need not be fixed to an individual
post. They can lawfully be fixed to railing or walls at the back
of the pavement. Pedestrian zone signs (sign 637 series) may
also be fixed to walls, etc. at the back of the pavement.
No entry signs
A single no entry sign is equally effective
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