Streets
One-way systems move cars past the shops that need them to stop
Converting streets to one-way raises vehicle capacity and speed, and quietly removes the conditions a high street depends on.
The options around one-way streets are set out side by side below, with the conditions that genuinely favour one over the other.
The difference in one place
- One-way conversion typically raises speeds by removing opposing flow and conflict.
- Total vehicle distance travelled rises because journeys become loops.
- Reversion to two-way has been pursued in a number of cities on economic rather than traffic grounds.
The capacity gain is real
Removing opposing traffic eliminates the need to hold back for oncoming vehicles and simplifies signal phasing at every junction on the route. That genuinely increases throughput on the corridor, which is why one-way systems spread widely during the decades when moving traffic was the objective. The gain is largest where the street was previously constrained by turning conflicts rather than by junction capacity.
None of this is disputed; what is disputed is what else the change does.
Speed rises because conflict falls
A driver with no oncoming traffic and no opposing turns has fewer reasons to slow and a wider effective carriageway to use. Multi-lane one-way streets also allow lane changing at speed, which is a pattern rarely seen on two-way urban streets. The resulting environment is measurably less comfortable for pedestrians crossing informally, which is most crossing on a shopping street.
On the ground, speed and severance are the mechanism by which a traffic improvement becomes a commercial problem.
Journeys become loops
A destination on a one-way street can only be reached from one direction, so drivers circulate to approach it correctly. That adds vehicle kilometres to the network even as it removes delay from the corridor, which is a genuine trade rather than a free gain.
It also makes the street network harder to hold in your head, and unfamiliar drivers make more late manoeuvres as a result. Delivery vehicles are affected disproportionately because they make many short stops.
Visibility to passing trade falls
A shop on a two-way street is passed by traffic in both directions; on a one-way it is passed by half as many movements. Drivers travelling faster also register fewer frontages, and the ones they do register are on their nearside only. Retail vacancy on one-way corridors has been raised repeatedly by business groups, though isolating the effect from wider retail decline is difficult and the evidence is not clean.
The mechanism is plausible and specific even where the quantification is contested.
Cycling suffers unless contraflow is provided
A one-way system forces cyclists into long detours or into riding illegally against the flow. Contraflow cycle lanes, or simply exempting cycles from the restriction on low-traffic streets, resolve most of this at modest cost. Several countries now apply a default presumption of cycle contraflow on one-way streets unless there is a specific reason against it.
At street level, where that presumption is absent, the one-way network becomes a barrier map for cycling.
Reversion is expensive and increasingly common
Returning a one-way system to two-way requires resignalling, remarking and often reconstructing junctions, so it is not a cheap correction. A number of cities have done it anyway, justified primarily on street-level economic activity rather than on traffic performance.
Post-conversion traffic outcomes have generally been less catastrophic than modelled, which points at the same adaptation effects seen elsewhere in traffic engineering. The lesson is that a corridor optimised for through movement was never optimised for the businesses along it.
Side by side
| Consideration | What it means in practice |
|---|---|
| The capacity gain is real | One-way conversion typically raises speeds by removing opposing flow and conflict. |
| Speed rises because conflict falls | Total vehicle distance travelled rises because journeys become loops. |
| Journeys become loops | Reversion to two-way has been pursued in a number of cities on economic rather than traffic grounds. |
The takeaway
Ask what the street is for. A one-way system answers that question whether or not anyone meant to.
Measure the walk, not the map.
Questions readers ask
Do one-way streets reduce congestion?
On the corridor itself, usually yes. Across the network the picture is mixed, because circulating traffic is displaced onto surrounding streets and total distance travelled rises.
Are one-way streets safer?
They remove head-on and turning conflicts and raise speeds. Studies have found effects in both directions depending on street type, so it is not a settled question.
