Leading Walk, Lagging Arrow

One of the easiest things you can do to help out pedestrians is the leading walk indication. At a normal traffic light, the pedestrian light turns to “walk” at the same time the signal head for cars turns green. This tends to lead to drivers trying to jump the light and turn right before the light turns green, cutting right in front of pedestrians as they start to cross the street – an experience you’ve surely had if you spend any time in downtown LA.

With a leading walk, the pedestrian light turns to “walk” a second or two before the signal for cars turns green. The idea here is that pedestrians get a chance to start crossing the street before the cars move, so drivers don’t get a chance to cut them off. This is really what’s supposed to happen anyway; we’re just tweaking the light to nudge people in the right direction. Obviously, this works even better if you have no right turn on red – something that’s appropriate during the day in a place like downtown LA where you have heavy pedestrian volumes. This arrangement is common in many places that have made a point to prioritize pedestrian movements, such as Cambridge, MA.

However, a problem can arise with this arrangement. If pedestrian volumes are very high, the crosswalk never clears, and it becomes very difficult for traffic to make turns. This decreases the capacity of the intersection and causes congestion, which is bad news for buses and emergency vehicles too.

The solution to this problem, if it arises, is hinted at by several lights around downtown LA that have a right turn arrow that comes up after the “flashing don’t walk” ends. This ensures some turning traffic gets to proceed during every light cycle, though it requires the pedestrian phase to be shorter than the maximum possible. A trade-off here might be to add some bulb outs at the intersection to increase the width of the sidewalks and crosswalks. While capacity of pedestrian facilities isn’t usually an issue, in this situation it might be, and this change would help improve pedestrian flow.


3 thoughts on “Leading Walk, Lagging Arrow

  1. Pingback: Today’s Headlines | Streetsblog Los Angeles

  2. Bjorn

    Leading pedestrian intervals are very common in Iowa City, and are even present on the four-lane state highway that bisects the city (to my surprise!). Without a no-turn-on-red restriction, which Iowa City appears to only use for geometric, not traffic flow reasons, what ends up happening is that drivers use the LPI interval to establish themselves in the intersection as pedestrians start to cross after coming to a stop (in theory), basically making a right turn on red. Sometimes drivers will make the turn anyway, especially if there are no pedestrians or if they are crossing from the far side of the intersection.

    I’m not sure that cutting off the pedestrian phase a few seconds early for a turn phase will actually work in practice. Enough pedestrians tend to cross trailing the ‘don’t walk’ flashing hand that a green arrow would merely complicate matters, resulting in turning traffic nudging themselves into the crosswalk on ‘their’ light, instead of yielding to pedestrians.

    I do like that the signals downtown are on a sixty-second cycle, which makes ‘missing’ a light less of a deal as one will only be waiting only approximately thirty seconds.

  3. alecpm

    This is exactly what we need more of downtown (though most intersections probably don’t need the lagging arrow and some one-way streets could probably use them for both left and right turners). It would also be nice to make the countdown timers operate during the full pedestrian phase (as in DC), giving walkers a clearer picture of the crossing time while they approach the intersection. Chinatown and the Fashion/Flower District especially need this sort of simple sensible traffic management which would be a tremendous benefit to the high volumes of both pedestrians and drivers there.


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