Traffic safety center at Transportation research board 2007
Timing Is Everything
The TSC Looks at Pedestrian Crossing Signals on a Busy Bay Area Thoroughfare
A major arterial in the East Bay is undergoing signal and system upgrades intended to reduce congestion and travel times for cars and trucks, as well as improve bus speeds and reliability (and, it is hoped, boost ridership).
But what about the pedestrians and bicyclists who also use the street?
That is the question Traffic Safety Center researchers are trying to answer as they assess the pedestrian and bicycle safety impacts of making a "Smart Corridor" along San Pablo Avenue, a four-lane roadway that runs on the east shore of San Francisco Bay from the City of Oakland some 20 miles north to the City of San Pablo.
Director David Ragland presented the center's findings from its initial study at the 2007 Annual Meeting of the Transportation Research Board (TRB). The study report, San Pablo Avenue Pedestrian Signal Timing Optimization, was written by TSC graduate student Anh Nguyen and Ragland.
San Pablo Avenue is of special interest to safety researchers, Ragland explained, because:
intelligent technology is used for signal control and traffic management,
traffic flow and speeds are important to maintain because the roadway serves as an alternative route for the often-congested I-80 freeway, which connects to the San Francisco Bay Bridge and two other freeways,
there is a complex mix of users due to proximity of small, residential streets, and a wide variety of land uses on contiguous properties,
the nearly uniform lane configuration and roadway design throughout its length, allows comparisons between locations, and
it is governed by seven jurisdictions.
As a first step in what will be a longer evaluation, the center's research team carried out an inventory of pedestrian signal timing to see if signals allowed pedestrians enough time to safely cross the street according to the prevailing standards of roadway design. In their TRB paper, Nguyen and Ragland explained how they chose to use the most conservative measure:
"There are two options for evaluating the sufficiency of pedestrian signal intervals for a given measured crossing distance. One option is the Total Pedestrian Interval (TPI), which is the sum of the “Walk” (W) interval plus the “Flashing Don't Walk” (FDW) interval. The other measure is a more conservative assessment, where the FDW interval is considered alone (also known as the “pedestrian-clearing interval”). This study places a greater emphasis on the FDW interval, which represents the minimum amount of time that a pedestrian can start and safely complete the crossing before the “Constant Don't Walk” (CDW) signal appears. " [Emphasis added.]
They also measured the actual distance from curb to curb at each signal and calculated the crossing time for each distance using the walking speeds set forth in the standard guidelines used in California and federal roadway design. They used two measures: 4 feet per second, which is for typical pedestrian locations, and 2.5 feet per second, which is for special land use areas, such as those near schools or senior citizen housing. They calculated the sufficiency of the crossing times for 295 crossings at 113 intersections.
For pedestrians walking at the standard speed, the flashing-don't-walk (FDW) interval was too short to cross in 60 percent of the crossings.
At the slower walking speed, the discrepancies were even greater: 95 percent of the FDW intervals were at least somewhat deficient. The deficiencies ranged from 48 seconds to 12 seconds.
"We have lots of deficiency by either definition and lots of variation across the corridor," Ragland said. Different cities had widely different rates of deficiency, with the larger cities tending to have greater shares of deficient signals.
"We don't know why this is, but we want to find out," Ragland said.
Another result that bears more study is the finding that, contrary to expectations, fewer signals for crossing San Pablo Avenue had excess time compared to crossings for the smaller side streets. Ideally, pedestrians would be given more time to cross the larger, busier street, which carries 20,000-30,000 vehicles a day. Ragland noted that this could be the result of signals having longer greens for through traffic on San Pablo, which would lead to longer pedestrian "walk" signals across the secondary roads.
Ragland concluded his TRB presentation with questions that he thought should be pursued:
"It is fine to have standards, such as 2.5 and 4 feet per second. How many are caught trying to cross? And what is the implication for how people view these crossings? Does it discourage people from walking? What is the implication for risk? We hope to be able to link some of these outcomes to some of these 295 crossings we looked at.
"Secondly, how did they get this way? What is the engineering? In many cases, the signal timing decision was made 20 years ago.
"Third, how do we assess what the best timing is? We have lots of land-use data— locations of schools, retirement homes, hospitals up and down this corridor. It would be interesting to see if these factors are correlated with signal timing."
Ragland also said the center would like to see if there is a correlation between signal timing and collisions. Roughly 80 pedestrian injuries occur on this corridor each year, such a small figure that it would be easier to understand if it were supplemented by more information, such as number of near-misses and conflicts that didn't result in crashes or injuries. As part of the Smart Corridor improvements, live video monitoring at roughly 70 intersections is now online. Ragland said the footage could prove valuable information for learning more about pedestrian/vehicle interactions. However, in its current form, it is too voluminous to study. He said the TSC was working on ways to automate analysis.
A "total needs assessment" is needed for the corridor, he said. If transit use increases, pedestrians will increase, too. And as buses and other vehicles move at greater speeds, safety must be made a priority.
—Phyllis Orrick