4 - Signal Control 2 Flashcards
What is the ‘y value’ ?
The ratio of flow to saturation flow for each stream
What is the ymax value?
The maximum value of y for each stage
What is the Y value?
Sum of y max for all stages
What is the optimum cycle time, C0?
C0 = (1.5L + 5) / (1-Y) where L is the lost time per cycle
As traffic growth takes place the capacity of a junction can be increased by increasing the cycle time, why is this?
Because the ratio of lost time to cycle time decreases
Why is a practical capacity of 90% of the max capacity time used?
To result in shorter delays
What does Y pract equal?
0.9 - 0.0075L
How do you calculate the reserve capacity of a junction using Ypract and Yex?
Reserve capacity (%) = 100(Ypract - Yexist) / Yexist
What is the 10 step procedure of a signal problem?
- Select phases and stages
- Obtain flows and convert to pcu flows appropriate for design year
- Calculate the y values for each stream
- Identify ymax for each stage
- Calculate the Y value
- Estimate the total lost time per cycle
- Calculate the optimum cycle time
- Calculate the green splits
- Check that the total of the green splits and the lost time is equal to the
cycle time - Calculate the reserve capacity of the junction
In a signal design question, you create a box with 7 headings, what are these 7 headings in order?
Stream
Stage
Approach
Direction
Flow
Sat Flow
y value
Without special provision, when can right turning traffic turn?
-In gaps in the opposing flow
-During the integreen period
When are early cut-offs used?
For heavy right turning flows from one direction only
When are an early cut off or late start used for the opposite phase?
-When the number of right turning traffic is not sufficient to justify the provision of right turning traffic, but, right turning vehicles have difficulty in completing the traffic movement.
What is an early cut-off?
When right turning traffic can complete their movement at the end of the green period when the oppsosing flow is halted. Other traffic are dtopped while one or more are allowed to keep moving.
What are the main problems with late starts?
Once a dominant flow has been established, those drivers who have initially been shown green assume an unopposed movement.
If there are right turning drivers on the oppsote flow, it can be difficult for them to make the movement.
If we have yW , YER and YE:
If yMAX W/E = yW + yER, what is y max for stage 1 and 2?
Stage 1 : yMax = yW
Stage 2: yMAX = yER
If we have yW , YER and YE:
If yMAX W/E = yE, what is y max for stage 1 and 2?
Stage 1: yMAX = (yW / yW + yER ) * yE
Stage 2 : yMAX = (yER / yW + yER ) * yE
