3 - Signal Control 1 Flashcards
What 2 things must junction design ensure?
-Road user safety
-Ensure capcity meets operational traffic-flow needs
What are 3 reasons for when signal control is necessary?
-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.
Signals are used to impose priority rules in order to:
-limit the number of simultaneuously potential vehicle confilcts
-give pedestrians a period without straight cross traffic.
What does ‘aspect’ mean?
The signal indication
What is the british signal sequence?
Red
Red and amber
Green
Amber
Red
What is the standard ‘red and amber’ duration?
2 seconds
What is the standard ‘amber’ duration?
3 seconds
What is a stream of traffic?
Traffic in a single lane analysed seperately, or in 2 or more adjacent lanes treated as a single queue
Why is 2 lanes with 2 streams a potential hazard?
Becuase of lane changing in the 2 streams.
What is a phase?
One or more stream receiving simultaneuously identical aspects
What is a stage?
The time period where one or more phases recieves green
What is a cycle?
One complete sequence of all the phases recieving green
What is a typical cycle time?
30 - 120s
Why are the cycle times in Nottingham around 60 seconds at max?
To favour pedestrians
What is the intergreen period?
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
What does the integreen period involve?
3 seconds of amber for the phase losing right of way
2 seconds of red/amber for the phase gaining right of way
What happens if the integreen period is too short?
Traffic is still clearing the junction
What happens if the integreen period is too long?
There is unneccessary delays
What is the minimum integreen period?
Usually 4 seconds
What does the amber period of phase A and the red/amber period of phase B overlap by?
1 seconds
On a wide, high speed road, how long is the integreen period?
7 seconds often, as there is 2 seconds of ‘all red’ for safety
What is the actual green, k ?
The duration of green aspect for a phase
What is G?
The combined green and amber, k + a
What is the saturation flow of a lane?
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
What is the saturation flow of an approach?
The sum of the saturation flows of the individual lanes comprimising the approach.
At the start of the green period, why is the discarge flow rate slower?
Due to vehicles accelerating to normal running speed
At the end of the green period, what happens to the discharge flow?
The discharge flow does not instantly decrease to 0
The rate falls from the saturation flow rate to 0 over the amber period.
Due to the differences in discharge flow rate, what do we replace green and amber time by?
An effective green period, throughout which discharge flow is assumed to take place at the saturation flow rate.
What is ‘lost time’?
Time at which no discharge flow takes place.
What is the difference between the effective green time and combined green and amber time typically?
2 seconds ( the lost time)
What is the Phase lost time, lp?
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
What equaton relates effective green time, g the sum of the green and amber period ( k +a ) and the lost time?
g = k + a - lp
What is the integreen lost time?
Li = I - a (all red + red + amber ) - amber time
What is the total lost time per cycle?
The sum of the phase lost time and the integreen lost time
sum of (lp + (I-a) OVER ALL STAGES
When 2 stages overlap, what is the integreen lost time?
0, as a stream runs through the integreen period
How is junction efficiency maximised?
If the number of stages is mimised, since the total lost time is minimised
What is the approach capacity?
The max amount of traffic that can pass through a signal controlled junction from a given approach
What does the approach capacity depend on?
The effective green time available to the traffic
The maximum possible flow that can cross the stop line during the green period gs/C
What is the degree of saturation, x?
The ratio of actual flow, q to the maximum flow that can pass through the approach
What is the equaton of the degree of saturation?
qC / gs where qC is the expected vehciles on approach, gs is the availability of the approach over the cycle
If the degree of saturation > 1 , what should happen?
The availabilty of the capacity should be increased.
