Wednesday, June 07, 2006

Pedestrian Scrambles and Signal Timing

Over at, we've been having a discussion about the relative merits of adding a flashing yellow cycle to left-turn signals. The yellow cycle would allow cars to make a left turn fromt he left-turn lane to the extent permitted by oncoming traffic; if traffic was too heavy, they would still get the normal left-turn-green arrow. Frank Dufay raised the issue, however, that the blinking yellow arrow, while it might be good for drivers, could raise the issue of additional pedestrian/vehicle conflicts in the crosswalk that vehicles turning on the blinking yellow would cross.

I didn't want to to muddy the issue too much, but I did want to ask how the pedestrian scramble (all-pedestrian light cycle period) might fit into the blinking-left-turn scenario.

For a while now, I've been of the belief that downtown Portland should re-time all of their traffic signals. I see two options:

One: Green lights would be for cars only, to allow for faster clearing of intersections and turn traffic. However, the green period would be shortened slightly, and a pedestrian scramble cycle inserted, during which no automobile movement would be allowed, but pedestrians could cross any direction they pleased.

Two: half of every green light period would give the OK to pedestrians. The other half would give a RED hand to pedestrians, to allow for turning movements of cars. There would then be an additional cycle, the pedestrian scramble, during which no automobile movement would be allowed, but pedestrians could cross any direction they pleased.

Perhaps downtown might not be the best first place to try this, (Broadway/Weidler or another basically stand-alone couplet might be more appropriate), but the reasons that I suggest it are these:

1. Pedestrian scrambles would seem to be the safest way to get pedestrians across the street. There are no turns on red allowed during the scramble cycle, so there is no potential for ped/vehicle conflict as long as everybody plays by the rules.
2. Initiating the scramble cycle at every intersection in a grid would allow for the entire grid to be re-timed at once. This would mean that, as you were driving along, you would still get greens as you progressed from one intersection to the next (assuming, of course, that you drove at the proper speed and congestion was not an issue). However, following behind the green cycle for cars would be an all-red cycle for cars -- the scramble cycle, when pedestrians would go for it.

I think there are two potential variations on this theme:

a) The lights cycle green for cars heading north/south, green for cars heading east/west, then the pedestrian scramble cycle.

b) The lights cycle green for cars heading north/south, then pedestrian scramble, then green for cars heading east west, then a second pedestrian scramble cycle!!

I've only ever seen the first option used in the real world. I'd love to see the second option tested in a high-pedestrian-volume area like downtown Portland!

Finally... bicycles. As I see it, bikes could go on either the appropriate green cycle for cars, or, if they were careful and slowed down, they could also traverse the intersection during the pedestrian cycle, if they did so at pedestrian speeds.

If pedestrian scrambles became the norm at every signalized intersection, then I think the blinking yellow light left-turning issue would become a moot pedestrian safety consideration. I think it's a great idea to reduce motorist dwell time and reduce congestion at left-turn signals.

Barring the pedestrian scramble, however, Frank does raise a good point about the added potential for ped/vehicle conflict.

1 comment:

Anonymous said...

I belive that the second option (All cars stop and people go Everywhere) is in use in Brisbane & Darwin Australia