This analysis reflects on data from 16 commercial office buildings collected between 2013 and 2015, through the enforcement of Arlington County’s transportation demand management site plan requirements. Mobility Lab and Arlington County Commuter Services set out to determine the relationship between parking and travel behaviors in the county’s transit-oriented Rosslyn-Ballston corridor, including what factors influenced employee commute mode choices.
Analysis of the 13 buildings included in the trip-generation portion of the study shows that the offices generated weekday daily trips at rates between 39 and 79 percent of the rates predicted in the Institute for Transportation Engineers Trip-Generation Manual. This held too for AM peak hour trips, as the building with the highest rate was still only 66 percent of the ITE prediction.
The study found parking occupancy in the corridor to consistently fall below the available supply: of the 16 buildings, the highest average weekday parking occupancy was only 85 percent of the maximum, and the lowest was found to be to 50 percent. This reinforces findings that many Arlington office buildings provide more parking than necessary, even when controlling for varying rates of occupancy in these buildings.
Only 3 percent of workers at the 16 buildings responded that they park on the street, rather than in a parking facility, demonstrating a low rate of spillover parking. Workers who drove parked overwhelmingly in the on-site garage and nearby public facilities.
Compared with all Arlington workers, study respondents drove alone to work less frequently (47 percent versus 54 percent of commute trips) and considerably less than the regional average (66 percent of commute trips). Respondents also used transit more than Arlington as a whole, (32 percent against 26 percent, respectively).
Employees in the corridor who received parking subsidies that reduced their out-of-pocket costs were much more likely to drive alone to work than those who paid full market value. Conversely, recipients of transit subsidies were about twice as likely to commute by transit. Meanwhile, subsidies for carpooling and vanpooling may have had a strong influence on those rates, but employees were so infrequently aware of them that the report authors could not analyze their correlation.
Approximately a quarter of respondents (24 percent) pay nothing to park in their office building. The report’s authors estimate the studied buildings offer $3.5 million annually in parking subsidies, which they extrapolate to suggest buildings in the Rosslyn-Ballston corridor may be offering $25 million each year in free parking. The estimate may conservative, given factors such as partial parking subsidies and the prevalence of free parking as reported in the State of the Commute regional survey.
When parking and transit subsidies are available together, the potential behavior impacts of the transit subsidy offer are diminished. When employers simultaneously make available free parking and subsidized transit, employees are half as likely to commute by transit as when they have access to transit subsidies but no parking subsidies.
Employees who report having carpool and vanpool benefits use them at rates similar to the use of other benefits. For example, 17 percent of employees who report access to carpool financial incentives take advantage of them, which is close to the 19 percent of employees who report using secure bicycle parking when they know it is available. This suggests a higher potential for carpooling adoption with greater promotion and awareness of the available benefits.
Planners should rely on locally collected trip-rate data instead of values from the Institute of Transportation Engineers’ Trip-Generation Handbook.
Given the low parking occupancy rates, the report suggests jurisdictions use local data to better tailor parking requirements to need and find ways to ease the regulatory process for converting underutilized parking for other uses.
Similarly, jurisdictions should encourage more shared parking, especially through the use of vertically mixed-use buildings that directly share garage spaces across land-use types.
Investments and strategies that help households downsize their car ownership may help them make more sustainable choices for commute trips.
The high walking, biking, and transit mode splits covered in the report should help frame debates on how to spend money on transportation investments and how to allocate space for different modes.
Jurisdictions should pursue strategies to discourage parking subsidies, which dampen the effectiveness of transit subsidies and other TDM programs.
- Short report version
- Public presentation slides
- Building variable data (.XLSX download)