Daily Archives: January 19, 2017

Do Pedestrian Scrambles Make Sense?

Pedestrian scrambles have enjoyed some increased popularity lately, popping up at Hollywood & Highland in LA and Brand & Harvard by the Americana in Glendale. This arrangement is also known as a Barnes dance after the traffic engineer who promoted it. Note that diagonal crossing is permitted at these locations.

There are several advantages to the scramble:

  • Pedestrians have their own phase, with no traffic movements, which decreases the likelihood of drivers crashing into pedestrians.
  • People wishing to cross both streets can do so in one crosswalk phase.
  • There are no pedestrians during the auto phases, which increases the throughput of the turning movements for cars.

However, there are also several disadvantages to the scramble:

  • If the scramble is added at the expense of left turn arrows, the capacity of left turn movements will be negatively impacted.
  • If the scramble is added but the other phases of the light cannot be shortened (i.e. the walk movement was not the constraining factor on phase length), the cycle time will increase.
  • Vehicular capacity in general may be reduced, simply by reducing the percentage of green time in the cycle (g/C ratio).
  • Pedestrians only wishing to cross one street may see an increased delay.

My hunch is that, in general, pedestrian scrambles only make sense where pedestrian volumes are very high. Otherwise, the increased pedestrian wait time (which encourages jaywalking) and reduced vehicular capacity may offset the benefits. Note that even the world-famous scramble in Shibuya only allows the shorter of the two diagonal crossings.

The reason scrambles make sense when pedestrian volumes are very high is that large pedestrian flows will crush vehicular turning capacity, especially for right turns, which rarely have their own turn arrow. When pedestrian volumes are low or moderate, all pedestrians can step off the curb at more or less the same time, leaving the tail end of the green cycle for right turns. When there are huge numbers of pedestrians, though, pedestrian flow will continue right until the end of the cycle, and almost no traffic can turn. This leads to dangerous turns by frustrated drivers and more congestion on city streets. Anyone who has spent time in Manhattan has probably noticed congestion fomented by vehicles stuck trying to turn – and nearly been clipped by a driver turning on their heels.

The scramble solves this by eliminating the conflict for turning vehicles; despite the lost green time in the cycle, this may improve vehicular capacity by preventing gridlock from forming. This is not the only way to address turning vehicle congestion; some traffic lights in downtown LA (east side of Figueroa at 6th and west side of Flower at Wilshire, for example) have a lagging right turn arrow that comes after the pedestrian interval clocks out.

In a future post, we’ll dive a little more into the weeds to compare these options.