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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