Travel Behavior: Commuter Response to the Fix I-5 Project

August 2010

Two years ago this summer, a one-mile stretch of Interstate 5 in downtown Sacramento was intermittently closed over a period of nine weeks for a $27 million reconstruction project. As “The Fix” kicked off, government and business leaders who were worried about congestion and the potential economic impacts of the closure actively promoted commute alternatives, including flex schedules, four-day work weeks, carpooling, transit and telecommuting. At the request of Governor Schwarzenegger, UC Davis research teams led by Civil and Environmental Engineering Professor Michael Zhang and Professor Emeritus Pat Mokhtarian launched a study to track and analyze traveler response. The goal was to learn from the experience and help transportation planners prepare for future fixes in an efficient way. The preliminary findings show auto trips dropped – but not as much as one might expect, and some commuters adopted permanent changes – but not attributable solely to The Fix.

“Our project became a real-life research lab for the rest of the country,” explains Mokhtarian. Given the nation’s aging infrastructure, major highway closures like The Fix are expected to occur more frequently. Highway planners elsewhere can prepare for how commuters might respond to such closures based on the ITS-Davis team’s research findings.

“Furthermore, what we learn about commuters’ inclinations to change habitual behavior in the context of this sort of incentive can potentially be transferred to other types of incentives such as improved transit service and different price signals,” she adds.

The project’s principal investigator, Michael Zhang, led a team of graduate students and postdoctoral researchers who collected field data from freeways, arterial streets, and transit services on traffic volumes, travel times, and transit ridership before, during, and shortly after the project. Mokhtarian surveyed commuters twice during the project, and once six months after the project ended.

The two halves of the study are complementary. Zhang’s analysis portrays the aggregate travel patterns on the ground, while Mokhtarian’s portion offers insight into the individual decisions leading to those aggregate outcomes. If traffic was lighter in the affected area during The Fix, was it because people chose other routes or other destinations, took vacation, worked at home, or abandoned the activity entirely? Were certain kinds of people more likely to make changes, and certain kinds of changes, than others?

Zhang’s findings show that although some routes saw significant reductions or increases in traffic, the total drop in the number of auto trips was only 3% to 6%, not as high as was anticipated. Instead of changing to alternative modes, such as light rail and bus transit, the majority of commuters and truckers who changed their travel patterns shifted to alternate routes, which caused significant delays on those detours. Some commuters turned to carpooling in the initial stages of The Fix, but reverted to solo-driving in the later stages, perhaps due to the minor impact of The Fix on the freeway routes with carpool lanes. Transit services saw a significant increase in their ridership between 2007 and the end of the Fix I-5 project, but analysis by graduate student Rachel Carpenter attributes this increase to rising gas prices. Light rail saw significant ridership increases, 10%-20% during The Fix, but bus ridership experienced only minor increases. One month after construction was completed, vehicle volumes on the roadway returned to pre-construction levels.

Mokhtarian’s findings from her survey taken six months after the project ended indicate that 41.6% have made a permanent change to the commute pattern they had before The Fix. Of those, however, two-thirds said The Fix had nothing to do with their change. Roughly 24% said The Fix was one factor. Only 9% said The Fix was the most important factor contributing to their permanent change.

In addition to the general findings, Mokhtarian, together with visiting scholars Liang Ye and Meiping Yun of Tongji University in Shanghai, China, chose to examine differences by gender. She hadn’t planned to take this research approach, but determined it would be a valuable opportunity to do so because there is very little literature on gender impacts of reconstruction projects.

The analysis showed women were more likely than men to report being impacted by The Fix – positively as well as negatively. For example, about 25% of women and 21% of men reported better travel speeds than usual during The Fix. The most common change in commute pattern either gender made was avoiding rush hour or taking a different route, but women were 5 to 8 percentage points more likely than men to do each of these things. Women were slightly more likely than men to alter their mode choices. And while they were more likely to increase transit or vanpool use, they were less likely to walk or bike. Mokhtarian summarized the findings to date last spring, at the first of a series of joint research webinars with Caltrans.

Two key findings from Zhang’s research stand out for transportation planners considering similar big construction projects that involve road closures. An adequate amount of redundant capacity in a transportation network is critical for handling large-scale disruptions to the network, since mode shift is not likely to be significant in the short run. When there is adequate redundant capacity, and with proper project timing and traffic management, fully closing a highway in one direction at a time is both practical and effective. A preliminary report of Zhang’s findings is forthcoming.