31-07-2012, 03:19 PM
TRAFFIC SIGNAL DESIGN
DESIGN.pdf (Size: 137.41 KB / Downloads: 214)
Overview
The con icts arising from movements of trac in dierent directions is solved by time sharing of the principle.
The advantages of trac signal includes an orderly movement of trac, an increased capacity of the intersection
and requires only simple geometric design. However the disadvantages of the signalized intersection are it aects
larger stopped delays, and the design requires complex considerations. Although the overall delay may be lesser
than a rotary for a high volume, a user is more concerned about the stopped delay.
Phase design
The signal design procedure involves six major steps. They include the (1) phase design
(2) determination of
amber time and clearance time,
(3) determination of cycle length,
(4)apportioning of green time,
(5) pedestrian
crossing requirements, and
(6) the performance evaluation of the above design. The objective of phase design
is to separate the con
icting movements in an intersection into various phases, so that movements in a phase
should have no con
icts. If all the movements are to be separated with no con
icts, then a large number of
phases are required. In such a situation, the objective is to design phases with minimum con
icts or with less
severe con
icts.
There is no precise methodology for the design of phases. This is often guided by the geometry of the
intersection,
ow pattern especially the turning movements, the relative magnitudes of
ow. Therefore, a trial
and error procedure is often adopted. However, phase design is very important because it aects the further
design steps. Further, it is easier to change the cycle time and green time when
ow pattern changes, where as
a drastic change in the
ow pattern may cause considerable confusion to the drivers. To illustrate various phase
plan options, consider a four legged intersection with through trac and right turns. Left turn is ignored. See
gure 41:1. The rst issue is to decide how many phases are required. It is possible to have two, three, four or
even more number of phases.
Two phase signals
Two phase system is usually adopted if through trac is signicant compared to the turning movements. For
example in gure 41:2, non-con
icting through trac 3 and 4 are grouped in a single phase and non-con
icting
through trac 1 and 2 are grouped in the second phase. However, in the rst phase
ow 7 and 8 oer some
con
icts and are called permitted right turns. Needless to say that such phasing is possible only if the turning
Four phase signals
There are at least three possible phasing options. For example, gure 41:3 shows the most simple and trivial
phase plan. where,
ow from each approach is put into a single phase avoiding all con
icts. This type of phase
plan is ideally suited in urban areas where the turning movements are comparable with through movements
and when through trac and turning trac need to share same lane. This phase plan could be very inecient
when turning movements are relatively low.
Figure 41:4 shows a second possible phase plan option where opposing through trac are put into same
phase. The non-con
icting right turn
ows 7 and 8 are grouped into a third phase. Similarly
ows 5 and 6
are grouped into fourth phase. This type of phasing is very ecient when the intersection geometry permits
to have at least one lane for each movement, and the through trac volume is signicantly high. Figure 41:5
shows yet another phase plan. However, this is rarely used in practice.
There are ve phase signals, six phase signals etc. They are normally provided if the intersection control is
adaptive, that is, the signal phases and timing adapt to the real time trac conditions.
DESIGN.pdf (Size: 137.41 KB / Downloads: 214)
Overview
The con icts arising from movements of trac in dierent directions is solved by time sharing of the principle.
The advantages of trac signal includes an orderly movement of trac, an increased capacity of the intersection
and requires only simple geometric design. However the disadvantages of the signalized intersection are it aects
larger stopped delays, and the design requires complex considerations. Although the overall delay may be lesser
than a rotary for a high volume, a user is more concerned about the stopped delay.
Phase design
The signal design procedure involves six major steps. They include the (1) phase design
(2) determination of
amber time and clearance time,
(3) determination of cycle length,
(4)apportioning of green time,
(5) pedestrian
crossing requirements, and
(6) the performance evaluation of the above design. The objective of phase design
is to separate the con
icting movements in an intersection into various phases, so that movements in a phase
should have no con
icts. If all the movements are to be separated with no con
icts, then a large number of
phases are required. In such a situation, the objective is to design phases with minimum con
icts or with less
severe con
icts.
There is no precise methodology for the design of phases. This is often guided by the geometry of the
intersection,
ow pattern especially the turning movements, the relative magnitudes of
ow. Therefore, a trial
and error procedure is often adopted. However, phase design is very important because it aects the further
design steps. Further, it is easier to change the cycle time and green time when
ow pattern changes, where as
a drastic change in the
ow pattern may cause considerable confusion to the drivers. To illustrate various phase
plan options, consider a four legged intersection with through trac and right turns. Left turn is ignored. See
gure 41:1. The rst issue is to decide how many phases are required. It is possible to have two, three, four or
even more number of phases.
Two phase signals
Two phase system is usually adopted if through trac is signicant compared to the turning movements. For
example in gure 41:2, non-con
icting through trac 3 and 4 are grouped in a single phase and non-con
icting
through trac 1 and 2 are grouped in the second phase. However, in the rst phase
ow 7 and 8 oer some
con
icts and are called permitted right turns. Needless to say that such phasing is possible only if the turning
Four phase signals
There are at least three possible phasing options. For example, gure 41:3 shows the most simple and trivial
phase plan. where,
ow from each approach is put into a single phase avoiding all con
icts. This type of phase
plan is ideally suited in urban areas where the turning movements are comparable with through movements
and when through trac and turning trac need to share same lane. This phase plan could be very inecient
when turning movements are relatively low.
Figure 41:4 shows a second possible phase plan option where opposing through trac are put into same
phase. The non-con
icting right turn
ows 7 and 8 are grouped into a third phase. Similarly
ows 5 and 6
are grouped into fourth phase. This type of phasing is very ecient when the intersection geometry permits
to have at least one lane for each movement, and the through trac volume is signicantly high. Figure 41:5
shows yet another phase plan. However, this is rarely used in practice.
There are ve phase signals, six phase signals etc. They are normally provided if the intersection control is
adaptive, that is, the signal phases and timing adapt to the real time trac conditions.