Transit
A bus lane is only as good as its last hundred metres
Lanes that end before the junction hand the bus back to the queue at exactly the point where the queue exists.
Comparisons of bus priority usually pick a winner. This one picks the circumstances, which is more useful.
The difference in one place
- Delay on a bus route concentrates at junctions, not on links.
- A bus lane that ends short of the stop line surrenders its benefit.
- Signal priority and queue jumps address the delay where it occurs.
Delay lives at the junction
On a congested corridor, most of the time a bus loses is spent in a queue approaching a signal rather than moving slowly between them. A bus lane along the link therefore delivers the bus to the back of the same queue slightly earlier.
The lane looks impressive on a map and delivers a fraction of its apparent benefit. Priority measures are worth building where the delay is, which means at and through the junction.
Lanes end for a reason and it is usually capacity
Bus lanes are commonly terminated well before the stop line to preserve turning capacity or to reinstate a general traffic lane. That decision is made to protect vehicle throughput and it directly cancels the priority the lane was built to provide. Where the lane does continue to the stop line, the bus arrives at the front and departs first.
The difference between these two designs on the same corridor can be substantial and is rarely reported separately.
Signal priority is cheap and underused
Detecting an approaching bus and either extending its green or bringing it forward removes delay without taking road space. Systems can be made conditional so priority is granted only to buses running late, which limits the cost to other traffic. The technology is mature and the obstacle is usually institutional, because signals and buses are frequently controlled by different organisations.
Where the same body controls both, priority tends to be implemented far more widely.
Queue jumps and gates use small amounts of space
A short bus-only lane on the approach with an early release signal lets the bus move ahead of the queue and rejoin in front of it. Bus gates — short sections closed to general traffic — achieve the same at a pinch point and are enforceable by camera. These require metres rather than kilometres of reallocation and are consequently much easier to deliver politically.
Their effect on reliability is often larger than their effect on average journey time, which matters more to passengers.
Enforcement determines whether any of it works
A bus lane occupied by parked, loading or through traffic is not a bus lane. Camera enforcement is the practical mechanism in most cities, and the legal basis for it varies by jurisdiction.
Loading restrictions in bus lanes are a common source of conflict with businesses, and time-limited arrangements are the usual compromise. Compliance data is generally available to the operating authority and is rarely published, which makes public scrutiny difficult.
What works in a dense grid may not transfer to a low-density suburb.
Reliability is the passenger-facing outcome
Passengers plan around the worst plausible journey time, not the average, so variability costs more than slowness. Priority measures reduce variability more than they reduce average time, because they remove the worst delays rather than shaving the good days.
This is why schemes evaluated only on average journey time understate their own benefit. The measure worth publishing is the spread of journey times, not the mean.
Side by side
| Consideration | What it means in practice |
|---|---|
| Delay lives at the junction | Delay on a bus route concentrates at junctions, not on links. |
| Lanes end for a reason and it is usually capacity | A bus lane that ends short of the stop line surrenders its benefit. |
| Signal priority is cheap and underused | Signal priority and queue jumps address the delay where it occurs. |
The takeaway
Follow the bus lane to its end. What happens there is the whole scheme.
Measure the walk, not the map.
Questions readers ask
Do bus lanes make congestion worse for other traffic?
They reduce general traffic capacity on the link, and effects vary with how much traffic shifts to the bus. Corridors have generally adapted more than fixed-demand models predicted.
What is conditional signal priority?
Priority granted only when a bus is behind schedule or above a certain occupancy, rather than to every bus. It targets the delay that matters while limiting the impact on other movements.
