Question 1 of 7: Signal-Phasing, Cycling and Traffic-Count Terminology

Question 1: Signal-Phasing, Cycling and Traffic-Count Terminology (20 marks — 4 each)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — leading versus lagging protected phase. A protected left-turn phase gives left-turning drivers an exclusive green arrow while the opposing through movement is held at red, so the turn is made with no conflicting vehicle stream. The words leading and lagging describe only when in the ring that protected interval is displayed relative to the through movement it shares the approach with. A leading protected phase is shown first: the arrow comes up at the start of the phase, the stored left-turn queue discharges, and the through movement is then released. A lagging protected phase is shown last: the through movement runs first, and the arrow is displayed after the through indication terminates.

Leading operation is the North American default because it matches driver expectancy and because it clears the left-turn bay before the through platoon arrives, which prevents the bay from spilling back into the adjacent through lane. Lagging operation is chosen when the intersection sits on a coordinated arterial: holding the arrow until the end of the phase keeps the leading edge of the through green undisturbed, so the progression band is not chopped, and it also suits protected-permissive operation where the permissive interval precedes the arrow. The cost of lagging is the left-turn trap: if the opposing approach carries a permissive left turn and its through indication terminates first, a permissive left-turner may read the opposing yellow as a signal that the opposing traffic has stopped when in fact it is still running under a lagging arrow. The trap is designed out either by using lead-lead phasing, by running both directions’ lefts protected, or by replacing the permissive circular green with a flashing yellow arrow that clearly conveys “yield to oncoming traffic” regardless of what the opposing head is showing.

Part (b) — protected versus permissive phase. The distinction here is not about ordering but about right of way. In a protected phase the turning movement has exclusive right of way — a green arrow is displayed and every conflicting vehicular and, ideally, pedestrian movement is held at red. In a permissive phase the movement is made under a circular green: the driver is legally permitted to complete the manoeuvre but must yield to the opposing through stream and to pedestrians in the conflicting crosswalk, so the turn is made in gaps that the opposing stream happens to offer.

The engineering consequences are large. A protected left turn discharges at close to a full saturation flow (of the order of 1,500–1,800 passenger cars per hour of green per lane), so its capacity is predictable and is set by the green time allotted. A permissive left turn has a capacity that depends entirely on the opposing flow: it is generous when the opposing volume is light, falls steeply once the opposing volume passes roughly 800–1,000 vehicles per hour, and collapses to the “sneakers” that clear on the yellow when the opposing stream is continuous. Safety follows the same pattern — the protected phase removes the crossing conflict entirely and is normally warranted where the left-turn volume is high, where the opposing approach has two or more through lanes, where sight distance to oncoming traffic is restricted, or where the collision history shows turning conflicts. The price of protection is an additional phase: every phase added contributes its own lost time, which lengthens the cycle and increases delay for all movements, so protection is bought at the expense of the rest of the intersection. Protected-permissive operation is the usual compromise, giving an arrow when the demand justifies it and a permissive interval to soak up the remaining turns.

Part (c) — bicycle lanes versus bicycle paths. A bicycle lane is part of the roadway. It is a longitudinal portion of the travelled way, delineated by pavement markings and signs, reserved for the exclusive use of cyclists, carrying one-way traffic in the same direction as the adjacent motor vehicle lane, and located between the general-purpose lane and the curb or parking lane. Typical Canadian widths are 1.5 m against a curb and 1.8 m or more beside on-street parking so that cyclists can ride clear of the door zone. Because it is part of the roadway, a bicycle lane is governed by the ordinary rules of the road, is swept and plowed with the roadway, and delivers cyclists to intersections where drivers are already looking for them.

A bicycle path — a shared-use path, or a cycle track where it is immediately adjacent to a street but physically separated from it — is not part of the roadway. It occupies its own right-of-way or is separated from the carriageway by a curb, a boulevard, a barrier or a row of parked cars; it is commonly two-way; and it is designed with its own alignment standards (3.0 m minimum width and 4.0 m where pedestrian use is expected, with its own design speed, stopping sight distance and horizontal curvature). The separation buys a large gain in perceived comfort and attracts less-confident riders, which is why the separated facility is now the default for arterials in Canadian active-transportation policy. Its weakness is exactly at the points where the separation ends: driveways and intersections, where a two-way path introduces contra-flow cyclists that turning drivers do not expect. Path crossings therefore need dedicated treatments — crossrides, bicycle signal heads, elephant’s-feet markings, and daylighting of the corner — which are set out in the MUTCDC and in TAC’s bikeway guidance.

Part (d) — HOV lanes. A high-occupancy-vehicle lane is a lane reserved, either at all times or during stated periods, for vehicles carrying at least a specified number of occupants — normally 2+ or 3+ — together with buses, and in some jurisdictions taxis, motorcycles and eligible low-emission vehicles. The design forms are the concurrent-flow lane (an inside lane of the freeway, separated from the general-purpose lanes by a painted buffer or a physical barrier), the contra-flow lane (borrowed from the off-peak direction, used where the directional split is very strong), and short local applications such as queue-jump lanes and bus bypass shoulders at ramps and signals.

The purpose of an HOV lane is to raise the person-throughput of a corridor rather than its vehicle throughput, so its performance measure is person-trips per hour and not vehicles per hour: a lane carrying 900 vehicles per hour at an average of 2.4 occupants moves more people than a general lane carrying 1,900 vehicles per hour at 1.15 occupants. The lane works only while it retains a genuine travel-time advantage, because that advantage is the entire incentive to carpool or to ride the bus; it must therefore be kept below capacity, which means it will always look emptier than the lanes beside it and will always attract political pressure to open it up. Enforcement is the other practical constraint, since occupancy is difficult to observe at speed and violation rates rise quickly if enforcement lapses. Canadian applications include the Highway 403 and Highway 404 HOV network in the Greater Toronto Area and the Highway 1 HOV lanes in Metro Vancouver.

Part (e) — cordon counts versus screenline counts. Both are area-wide volume studies, and they differ in the geometry of the counting boundary and therefore in what the data can be used for. A cordon count uses a closed boundary drawn around a defined area — a central business district, a campus, a hospital precinct — and every point at which a street crosses that boundary is counted in both directions over the study period. Because the boundary is closed, the running difference between inbound and outbound crossings gives the accumulation inside the cordon at any instant, which is exactly the quantity needed for parking-supply studies, for downtown travel-demand-management programs and for evaluating a cordon-pricing scheme.

A screenline count uses an open line that cuts across the study area, normally following a natural or built barrier such as a river, a rail corridor or a ravine, so that the number of crossing points is small and can be counted economically. It yields directional volumes across that line but says nothing about accumulation, because traffic can enter and leave the study area without ever crossing it. Its classic use is calibration and validation of a travel-demand model: assigned trips crossing the screenline are compared with counted trips crossing it, and a systematic discrepancy points to an error in trip generation, distribution or assignment rather than to a local count problem. In short — a cordon is a closed loop that measures what is inside; a screenline is an open cut that measures what moves across.