Junction Capacity Is The Reason A Street Is Wide

A road's capacity is set at its junctions, not along its length. Yet the width required to feed those junctions is frequently carried through the whole street.

Junctions ration flow and links do not

Traffic moves freely along a link until it reaches a point where it must give way, merge or wait for a signal, and that is where queues form.

The number of vehicles a corridor can carry is therefore determined by its most constrained junction, regardless of how many lanes run between them.

Adding capacity mid-block does nothing for throughput. It simply allows vehicles to reach the back of the same queue sooner.

Approach lanes create the width

Junctions need separate lanes for turning movements so that a vehicle waiting to turn does not block traffic going straight on.

Those lanes require a taper and a storage length sufficient to hold the expected queue, which can extend a considerable distance back from the stop line.

Where two junctions are close together, the flares from each meet in the middle, and the street is wide along its entire length for reasons that exist only at its ends.

The surplus width is rarely reallocated

Mid-block carriageway that is not needed for capacity could become footway, cycle track, planting or parking without affecting how much traffic the corridor carries.

Reallocation is resisted because width reads as capacity, and removing it appears to reduce the network even where the analysis says otherwise.

It is also administratively harder than leaving things alone, since changing a road layout requires consultation, a traffic order and a construction budget.

Wide links produce speed between the queues

Drivers accelerate where the road looks open and brake at the junction, producing a stop-start pattern that is worse for noise, emissions and collision severity than steady flow.

The perceived openness comes from width and from the absence of anything at the edges, which is precisely what surplus carriageway provides.

Narrowing the link therefore reduces speed without reducing throughput, because throughput was never limited there.

Junction redesign is where the real trade is made

Reducing the number of approach lanes cuts the width required and shortens the crossing distance for pedestrians, at a cost in vehicle delay.

That trade is explicit and measurable, which makes it politically visible in a way that mid-block width never is.

Cities that have narrowed streets successfully generally started at the junctions, because everything else about the street's width follows from what happens there.