CIVL2700 · Transport Systems
Signal Timing I: Phasing, Clearance & Lane Groups
This Week-10 topic of CIVL2700 Transport Systems at the University of Sydney turns a signal plan into numbers. You build the phases, size the between-phase clearance with the ITE yellow y = τ + v/(2a) and all-red AR = (w + l)/v, test for a dilemma zone, and read the saturation flow to split each approach into lane groups. Taking the critical (maximum v/s) lane group per phase gives the sum of critical flow ratios Yc and the total lost time L — the two inputs the Webster cycle uses in the next chapter.
What this chapter covers
- 01Phasing recap: a phase serves non-conflicting movements; 2-5 phases; protected turn phase from the cross-product warrant
- 02Yellow (amber) time y = τ + v/(2a + 2Gg); level grade gives y = τ + v/(2a); τ ≈ 1.0 s, a ≈ 3.05 m/s²
- 03All-red time AR = (w + l)/v with w = width to clear and l = vehicle length (≈ 6 m); intergreen I = y + AR
- 04Speed converted to m/s (km/h ÷ 3.6); both y and AR rounded UP to the nearest 0.5 s
- 05Grade sign: a downgrade (G negative) lengthens the yellow; an upgrade shortens it; grade does not enter AR
- 06Dilemma zone: exists when the amber is shorter than ψmin = τ + v/(2a) + (W + L)/v, so a driver can neither stop nor clear
- 07Saturation flow s = 3600/h from the settled saturation headway h; higher for through than turning lanes
- 08Effective green g = G + I − l, effective red r = R + l, cycle C = g + r; movement capacity c = s·g/C
- 09Lane groups: same-lane movements together, exclusive turn lanes separate; flow ratio v/s = arrival flow / saturation flow
- 10Critical lane group = maximum v/s in a phase; Yc = Σ(v/s)ci and L = Σ(tL)ci, one per phase, need Yc < 1
Yellow & all-red at a 50 km/h level approach (ITE)
- +1Convert the speed to m/s: v = 50/3.6 = 13.89 m/s (the ITE clearance formulae use m/s).
- +1Yellow (level grade, so the grade term is 0): y = tau + v/(2a) = 1.0 + 13.89/(2 x 3.05) = 1.0 + 2.28 = 3.28 s, round up to 0.5 s -> y = 3.5 s.
- +1All-red: AR = (w + l)/v = (24 + 6)/13.89 = 30/13.89 = 2.16 s, round up to 0.5 s -> AR = 2.5 s.
- +1Intergreen: I = y + AR = 3.5 + 2.5 = 6.0 s of clearance between this phase and the conflicting one.
Key terms
- Phase
- The part of a signal cycle during which a chosen set of non-conflicting movements has right of way. A cycle is the full sequence of phases, typically 2 to 5.
- Intergreen (clearance interval)
- The yellow plus all-red between one phase ending and the conflicting phase starting: I = y + AR. It lets committed vehicles get clear of the intersection.
- Yellow (amber) time
- The ITE clearance y = tau + v/(2a + 2Gg); on the level y = tau + v/(2a). Rounded up to the nearest 0.5 s; a downgrade (G negative) lengthens it.
- All-red time
- The interval AR = (w + l)/v when every approach shows red, sized so a vehicle just entering can cross width w. Rounded up to 0.5 s; grade does not affect it.
- Dilemma zone
- A stretch of road, when the amber is too short, where a driver can neither stop safely before the line nor clear the junction in the amber. Avoided when amber >= tau + v/(2a) + (W + L)/v.
- Saturation flow (s) & effective green (g)
- s = 3600/h is the maximum discharge a lane sustains during a fully saturated green (settled headway h); the effective green g = G + I - l is the time that flow is assumed to occur, giving movement capacity c = s*g/C.
- Lane group
- A set of lanes analysed together: same-lane movements form one group and an exclusive turn lane is its own group. Each has a flow ratio v/s.
- Critical lane group / flow ratio
- In a phase, the lane group with the highest flow ratio v/s (arrival flow / saturation flow); it sizes that phase's green. Yc = sum of the critical v/s, one per phase.
Signal Timing I: Phasing, Clearance & Lane Groups FAQ
Why are the yellow and all-red rounded UP to 0.5 s and never down?
Because they are safety clearances: the yellow gives a driver time to stop or commit, and the all-red flushes committed vehicles before the conflicting green. Rounding down would shorten that protection and could create a dilemma zone, so both y and AR are rounded up to the nearest 0.5 s - and each is rounded before you add them for the intergreen I = y + AR.
What is the difference between displayed green and effective green?
Displayed green G is what the driver sees; effective green g = G + I - l is the time the saturation flow is actually assumed to occur. Discharge does not start the instant green shows (a start loss) and runs a little into the yellow (an end gain), so capacity uses the effective green, c = s*g/C, not the displayed green.
Can AI help me with signal timing, clearance and lane groups in CIVL2700?
Yes - Sia can explain the ITE yellow and all-red formulae step by step, check that you converted speed to m/s and rounded up to 0.5 s, and walk through how the critical lane group's v/s builds Yc and the total lost time on your own practice numbers. It is a study aid that explains the method; it will not sit your assessment or guarantee a mark, so always confirm final answers against your Canvas materials.
Exam move
Drill the mechanical order every time: convert the approach speed to m/s (km/h divided by 3.6), apply the ITE yellow y = tau + v/(2a) (add the grade term only if the approach is not level) and all-red AR = (w + l)/v, round each up to 0.5 s, then add for the intergreen. Be able to explain a dilemma zone and test whether the amber beats the minimum clearance psi_min. For the lane-group thread, write the saturation flow s = 3600/h, form each lane group's v/s, take only the critical (maximum) one per phase, and sum to Yc with the total lost time L = one start-up plus one clearance per critical group - showing the working, since a bare Yc rarely earns full marks. The final exam is a 2.5-hour, 40% hurdle task, so budget time in proportion to the marks on each part and confirm the exam's open- or closed-book status and exact timing in the June 2027 Semester-1 exam period on Canvas.
Working through Signal Timing I: Phasing, Clearance & Lane Groups in CIVL2700? Sia is AskSia’s AI Engineering tutor — ask any CIVL2700 Signal Timing I: Phasing, Clearance & Lane Groups question and get a clear, step-by-step explanation grounded in how CIVL2700 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
