Tools for Operational Planning: A new group of simulation tools will allow engineers to quickly diagnose sick transportation corridors, then prescribe the best fixes for congestion, bottlenecks, and other delays on freeways and surrounding arterials.
Drivers in the Bay Area are no strangers to freeways where traffic suddenly comes to a standstill, or to major thoroughfares where only a handful of vehicles make it through an intersection before the light turns red leaving dozens of weary commuters piled up behind the signal. For these familiar situations, Berkeley residents need look no further than I-80 and its arterial, San Pablo Avenue.
Traffic engineers have experienced the same frustrations. And while it may seem like congestion shouldn't be so difficult to solve, consider this: for every action there is an equal reaction.
Imagine, for example, a rubber glove partially filled with water and sealed on top. Squeeze the rubber glove's thumb, and the other fingers swell.
Traffic is a little like water; adjustments made to a traffic problem in one area will produce a reaction in another.
A frustrated commuter stuck in first gear on I-80 in Berkeley may decide it's faster to exit the freeway and head north or south on San Pablo Avenue. So will a lot of other freeway commuters, and the result will be a slowdown on San Pablo, where signals timed to work in normal traffic fail under more congested conditions, causing back ups.
But solutions are on the way. With voter approval in 2006 of a $20 billion bond measure to improve transportation, Caltrans launched an ambitious "corridor management program," with an equally ambitious goal: to reduce congestion by 40 percent by 2025.
In response, researchers at ITS Berkeley’s PATH (California Partners for Advanced Highways and Transit) have constructed a toolbox of computer simulation programs called TOPL, or Tools for Operational Planning, that will ultimately allow transportation engineers to manage traffic flow better, not only on freeways, but on equally congested nearby surface roads.
TOPL is a so-called “macro” model, which can model and analyze freeway and roadway traffic flow rates. In addition, TOPL “is the only model that works directly with PeMS data on freeways,” explained PATH Director Alex Skabardonis.
“Using a micro model for a 20-mile corridor could take hours to run. TOPL takes seconds.”
PeMS, or Freeway Performance Management System, was also developed by PATH researchers. Using sensors, PeMS collects historical and real-time freeway data from freeways throughout the state to compute freeway performance measures.
By utilizing PeMS, TOPL minimizes data input and calibration requirements when applying the model, which in turn “reduces tremendously the effort involved,” Skabardonis added. “Using a micro model for a 20-mile corridor could take hours to run. TOPL takes seconds.”
The simulation programs that make up TOPL allow its users to examine discrete portions of a freeway and its surrounding surface streets to predict how best to solve specific congestion problems without creating greater congestion elsewhere along this particular traffic corridor.
One TOPL tool is Aurora, which uses PeMs and other data from Caltrans to provide an accurate simulation of traffic conditions on I-80 at various times of the day. Transportation planners can examine how different congestion-busting strategies might speed up traffic. What would happen if
- another freeway lane were added?
- ramp metering were installed?
- speeds on freeways could be controlled by traffic management?
- variable message signs could direct drivers to exits in order to avoid accidents?
- traffic signals on nearby arterials could be adjusted to accommodate a greater number of vehicles when freeway traffic slows?
TOPL simulators also offer a chance for traffic engineers to alter "reality" by adding crashes, sporting events, or a sudden downpour.
Which strategies might provide the best solution? TOPL simulators also offer a chance for traffic engineers to alter "reality" by adding crashes, sporting events, or a sudden downpour—all of which affect traffic flow—to the model to see where congestion occurs and what might prevent it .
PATH research engineer Andy Chow, explains: "We know, for example, that on sections of I-80W where the normal speed limit is 55 mph, when drivers must begin weaving to change lanes at the Powell Street on-ramp, congestion builds and quickly results in stop-and-go traffic. But if we were able to limit speed to 40 mph, we could facilitate drivers who need to switch lanes and keep traffic moving instead of deteriorating to a stop-and-go situation."
The TOPL group of engineers has been developing the I-80 / San Pablo integrated corridor model since 2008. They have already completed a model for I-210 in southern California.
"We are currently studying the demand and signal timing data of San Pablo Avenue that we just received from Caltrans," Chow added. According to Chow, the corridor will eventually include:
- a calibrated I-80 freeway model using PeMS data for both directions, from the Bay Bridge to the Carquinez Bridge;
- an arterial model of the entire length of San Pablo Avenue, from Oakland to the Carquinez Bridge, and
- a model of the major connecting streets, that link the freeway and the arterials. An example would be University Avenue.
When completed, the TOPL toolbox will allow transportation engineers to determine the best way to keep traffic moving—not only along the freeway but also along parallel arterials and connecting streets.
“Recent demonstrations show strong interest by practitioners,” Skabardonis reported. “It is possible that TOPL will be used in the Bay Area to analyze traffic management strategies as early as next year.”
For more information on TOPL: