[TCUG] Re: Late Start Signal Sequence

  • From: "David Overton" <dtoverton@xxxxxxxxxxxxxx>
  • To: <tcug@xxxxxxxxxxxxx>
  • Date: Sun, 20 May 2007 17:55:00 +0100

There seems to be some confusion in this thread.  TA16/81 has now been
replaced by TAL1/06 which is due to be incorporated into DMRB as TA 16/07.
TAL1/06 takes on board the descriptions of the three distinct uses of green
arrows (The directional arrow shown instead of a full green, the filter
arrow shown with a red and the indicative arrow shown with a full green) and
describes late start and early cut-off signalling.

The staging described by Peter Jones:

"Do you mean a two-way stage followed by closing one direction to leave
the opposite with a right turn arrow brought in by say a call cancel

is an Early Cut-off.

TAL1/06 defines Late Start (also known as "Late Release" and, confusingly,
"Early Start") as:

"a condition in which one or more traffic streams are permitted to move
before the release of other traffic streams which are permitted to run with
them during the subsequent stage".

It goes on to say:

"This method of operation is not recommended. It displays an indicative
arrow to one approach whilst delaying the start of the opposing traffic. The
two problems are: once a dominant flow has been established, those drivers
having been initially shown a green arrow assume an unopposed exclusive
movement and for the right-turning driver on the opposing flow, it is
difficult to make the movement and unnecessary risks are taken."

The history of Late Start staging and its association with the Northwest is
interesting.  It originated in the days when the then Ministry of Transport
was responsible for all signal design and the MoT NW signals engineer,
(Wally Thom by name if I remember correctly) was very keen on the practice
and had them installed in great numbers even at junctions where an early
cut-off would never have been considered.  Often a fixed two or four second
late start would be included in every cycle to allow just one or two
vehicles to turn right before the opposing traffic.

There are several advantages for the Late Release system.

1.  Because traffic can turn right during the late release period as well as
in the intergreen after the main stage, the average number of vehicles
waiting to turn right is lower than with early cut-off.

2.  There is not the same problem for the opposite right turn (and sometimes
pedestrians) which occurs with early cut-off staging and which requires the
use of closely associated secondary signals.

3.  The potential for serious accidents between right turners and opposing
traffic (which can occur with early cut-off and late start as the stage
changes from one-way to two-way or vice versa) is less for late start than
for early cut-off.  This is because in late start the last right turner is
in potential conflict with the first opposing straight-on vehicle which is
starting from rest; in early cut-off the first right turner is potential
conflict with the last opposing straight on vehicle which is travelling at
full speed (and may even be accelerating to beat the amber at the end of its
green phase).

The problems with Late Start signalling arise from the transition between
the protected right turn stage to the two way traffic stage.  Without an
indicative green arrow, right turners have no information that they have a
protected right turn or when that protection has been removed and opposing
traffic has received a green signal.

I was unfamiliar with Late Start signalling when I moved to Manchester in
1980 to head up the Greater Manchester UTC Unit.  Having suffered an
accident to my own (new) car when turning right at one such junction on my
first visit to the NW, I set about removing unnecessary late start stages
and replacing others with early cut-off.  However accident records showed
that total PI accidents (and particularly accidents involving right turners)
actually increased after conversion.  It is possible that, because of
advantage (3) above, this was caused by unrecorded damage only accidents
being replaced by personal injury accidents.  I also discovered that a
little-quoted TRL research report on accidents at four-way signals also
concluded that late release signals had a lower accident rate than early
cut-off ones.  The conversion programme was halted and instead existing late
start signals were equipped with green indicative arrows which were
extinguished at the end of the protected right turn stage.  I would have
preferred to follow the arrow with its own associated amber for three
seconds but this was not acceptable to whatever DTp was known as at the time
(even though this had been successfully tried in Northern Ireland).

Anyway, the use of green arrow with late start was successful and has now
become the accepted way of signalling late starts as can be seen from the
comment in TAL1/06.  Late release (with green arrows) remains popular with
Northwest signals engineers and drivers alike.  The danger of using it where
it is not currently normal practice is that the green indicative arrow might
be assumed to be always associated with early cut-off (or separately staged
opposing flows) and drivers amy assume that their protection will remain
until the end of the stage when the amber appears even though the green
arrow is extinguished.

David Overton

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