Streets
Left Turns Cost More Than Any Other Movement
A turn across oncoming traffic consumes signal time, creates the most dangerous conflict points and drives intersection width, which is why designers try to remove them.
An intersection's size, timing and safety record are shaped disproportionately by one movement. Turning across opposing traffic is the maneuver that costs the most in every dimension.
The conflict geometry is the worst available
A vehicle turning left crosses the path of oncoming traffic and then crosses a crosswalk, so it conflicts with two streams in a single movement.
The driver is also judging the speed of an approaching vehicle while accelerating, which is a harder perceptual task than any other routine maneuver.
Pedestrians in the receiving crosswalk are frequently behind the driver's attention, since the driver is watching the oncoming gap rather than the destination.
Signal time is the scarce resource
Giving left turns a protected phase means stopping everything else, so every second allocated to it comes out of another movement's green time.
Permitted turning, where drivers find their own gaps, saves that time but reintroduces the conflict, which is the trade the timing engineer is making.
Because pedestrian crossing time is often the residual after vehicle phases are allocated, left turn phases indirectly shorten how long people have to cross.
Turn lanes decide intersection width
Storing turning vehicles requires a separate lane so they do not block through traffic, which widens the intersection by a full lane in each direction.
That widening lengthens every crosswalk, increases exposure time for people walking, and consumes land at the corner where buildings would otherwise sit.
An intersection therefore grows to accommodate a minority of the movements passing through it, and the resulting width applies to everyone.
Designs that remove the movement
Several intersection types redirect left turns rather than accommodating them, sending drivers past the intersection to turn elsewhere and rejoin.
Roundabouts eliminate the crossing conflict entirely by converting all movements to merges, though they need space and change pedestrian crossing arrangements.
Delivery and freight operators have applied the same logic to routing, minimizing left turns across a whole route to reduce idling and risk.
Where the constraint shows on a network
Restricting turns at busy intersections pushes those movements onto side streets, which is how a residential block acquires traffic it did not generate.
One-way pairs and median treatments manage this by concentrating turns where the geometry can handle them, at the cost of longer trips.
Design standards for these treatments come from state and national engineering guidance adopted locally, and the applicable version differs by agency.
Questions readers ask
Why do people cross against the signal when a crossing is right there?
Usually because the wait is long relative to the gaps in traffic. Behaviour tracks the cost of waiting far more closely than it tracks the rule.
Are staggered crossings safer than single-stage ones?
They simplify vehicle movement and shorten each exposure, but they lengthen the total wait and add a turn. The evidence depends heavily on the street, and neither form is better everywhere.
