3 - Signal Control 1

Question 1

What 2 things must junction design ensure?

Answer 1

-Road user safety
-Ensure capcity meets operational traffic-flow needs

Question 2

What are 3 reasons for when signal control is necessary?

Answer 2

-without it, some flow options would be impossible due to conflicts with other traffic movements
-without it, the junction becomes unsafe
-there is significant pedestrian traffic.

Question 3

Signals are used to impose priority rules in order to:

Answer 3

-limit the number of simultaneuously potential vehicle confilcts
-give pedestrians a period without straight cross traffic.

Question 4

What does ‘aspect’ mean?

Answer 4

The signal indication

Question 5

What is the british signal sequence?

Answer 5

Red
Red and amber
Green
Amber
Red

Question 6

What is the standard ‘red and amber’ duration?

Answer 6

2 seconds

Question 7

What is the standard ‘amber’ duration?

Answer 7

3 seconds

Question 8

What is a stream of traffic?

Answer 8

Traffic in a single lane analysed seperately, or in 2 or more adjacent lanes treated as a single queue

Question 9

Why is 2 lanes with 2 streams a potential hazard?

Answer 9

Becuase of lane changing in the 2 streams.

Question 10

What is a phase?

Answer 10

One or more stream receiving simultaneuously identical aspects

Question 11

What is a stage?

Answer 11

The time period where one or more phases recieves green

Question 12

What is a cycle?

Answer 12

One complete sequence of all the phases recieving green

Question 13

What is a typical cycle time?

Answer 13

30 - 120s

Question 14

Why are the cycle times in Nottingham around 60 seconds at max?

Answer 14

To favour pedestrians

Question 15

What is the intergreen period?

Answer 15

The time from the end of the green period of the phase losin right of way to the start of the green period of the phase gaining right of way

Question 16

What does the integreen period involve?

Answer 16

3 seconds of amber for the phase losing right of way
2 seconds of red/amber for the phase gaining right of way

Question 17

What happens if the integreen period is too short?

Answer 17

Traffic is still clearing the junction

Question 18

What happens if the integreen period is too long?

Answer 18

There is unneccessary delays

Question 19

What is the minimum integreen period?

Answer 19

Usually 4 seconds

Question 20

What does the amber period of phase A and the red/amber period of phase B overlap by?

Answer 20

1 seconds

Question 21

On a wide, high speed road, how long is the integreen period?

Answer 21

7 seconds often, as there is 2 seconds of ‘all red’ for safety

Question 22

What is the actual green, k ?

Answer 22

The duration of green aspect for a phase

Question 23

What is G?

Answer 23

The combined green and amber, k + a

Question 24

What is the saturation flow of a lane?

Answer 24

The maximum flow, in pcu/hr of green time that can be discharged from the lane where there is a continuous green aspect and flow of vehicles

Question 25

What is the saturation flow of an approach?

Answer 25

The sum of the saturation flows of the individual lanes comprimising the approach.

Question 26

At the start of the green period, why is the discarge flow rate slower?

Answer 26

Due to vehicles accelerating to normal running speed

Question 27

At the end of the green period, what happens to the discharge flow?

Answer 27

The discharge flow does not instantly decrease to 0 The rate falls from the saturation flow rate to 0 over the amber period.

Question 28

Due to the differences in discharge flow rate, what do we replace green and amber time by?

Answer 28

An effective green period, throughout which discharge flow is assumed to take place at the saturation flow rate.

Question 29

What is 'lost time'?

Answer 29

Time at which no discharge flow takes place.

Question 30

What is the difference between the effective green time and combined green and amber time typically?

Answer 30

2 seconds ( the lost time)

Question 31

What is the Phase lost time, lp?

Answer 31

The time that is effectively lost to traffic movement during the phase due to: -starting delays -falling off of discharge flow rate during the amber period -lp = 2 seconds typically

Question 32

What equaton relates effective green time, g the sum of the green and amber period ( k +a ) and the lost time?

Answer 32

g = k + a - lp

Question 33

What is the integreen lost time?

Answer 33

Li = I - a (all red + red + amber ) - amber time

Question 34

What is the total lost time per cycle?

Answer 34

The sum of the phase lost time and the integreen lost time sum of (lp + (I-a) OVER ALL STAGES

Question 35

When 2 stages overlap, what is the integreen lost time?

Answer 35

0, as a stream runs through the integreen period

Question 36

How is junction efficiency maximised?

Answer 36

If the number of stages is mimised, since the total lost time is minimised

Question 37

What is the approach capacity?

Answer 37

The max amount of traffic that can pass through a signal controlled junction from a given approach

Question 38

What does the approach capacity depend on?

Answer 38

The effective green time available to the traffic The maximum possible flow that can cross the stop line during the green period gs/C

Question 39

What is the degree of saturation, x?

Answer 39

The ratio of actual flow, q to the maximum flow that can pass through the approach

Question 40

What is the equaton of the degree of saturation?

Answer 40

qC / gs where qC is the expected vehciles on approach, gs is the availability of the approach over the cycle

Question 41

If the degree of saturation > 1 , what should happen?

Answer 41

The availabilty of the capacity should be increased.