Tolling strategies may mitigate the impact of AVs on VMT and congestion
By Michael Brenneis
The anticipated shift to autonomous vehicles raises several concerns, among them are whether AVs will increase total vehicle miles traveled, exacerbate congestion, or replace the use of transit and active modes. A new study focused on Austin, TX, models the effects that two adoption scenarios may have, comparing the results to current conditions. The experiment shows a jump in VMT, increased congestion, and a shift away from public transportation. The authors then mitigate the scenarios by applying four tolling schemes, and examine the results.
If AV adoption rates are high, road capacity may initially increase since AV connectivity would permit them to operate closer together, at least until the ease of traveling in an AV encourages more driving. AV users may be willing to travel farther, and spend more time in vehicles, because they can use time in the vehicle to do other things. For those conventional drivers remaining on the road as this transition occurs, travel times could increase, making conventional driving even more unattractive.
The modelers examined three scenarios: widespread adoption of private AVs; widespread use of shared autonomous vehicles (SAVs); and a baseline scenario with no replacement of cars by AVs, reflecting current conditions. According to the researchers it is possible that AVs would replace privately owned vehicles or be adopted in a shared capacity.
The study employed MATSim software, a multi-agent transportation simulation capable of modeling reactions to demand management strategies, including modified start times, trip duration, and destination shuffling, in an effort to calculate results more realistic than traditional single-trip models. Model parameters have generally been derived from previous work, and further calibration of model parameters is left to future work, although the authors argue that their parameters are justifiable and have produced reasonable results. The SAV model minimized the size of the fleet needed to serve all users, and utilized dynamic routing to more realistically represent conditions.
Figure 1. Mode split generated by the three scenarios.
Table 1. VMT and delay generated by AV and SAV use.
The simulated mode split for the three scenarios (Fig. 1) shows increases to both VMT and travel delay (Table 1.) A reduction in the proportion of trips made by walking/biking or public transit is also evident, down from 16 percent to 6 and 12 percent respectively. Some of the mode shift observed in this study may be attributable to a lack of transit or active mode options positioned to replace auto travel.
Conventional congestion pricing tends to be facility-based, including tolls on bridges, tunnels, and highways. Four tolling schemes were modeled including a link-based approach that applies tolls to the most congested road segments during peak travel time, and a distance-based toll applied ($0.10 per mile) between the hours of 7AM and 8PM to all roads. These are both considered traditional forms of congestion mitigation.
The connectivity of AVs allows tolls to be dynamically applied to each road segment according to traffic conditions. This is referred to as the marginal cost pricing scheme (MCP). The other advanced tolling strategy applied in the study is the travel-time congestion scheme, which charges drivers for the network level delay that they cause. It costs more to travel at times of peak demand on congested roads, similar to ridesharing companies’ surge pricing.
Figure 2. Results of tolling schemes on VMT and congestion.
As one might expect, all of the tolling scenarios result in reduced VMT and delay (Fig. 2). The AV scenario sees the most VMT reduction under the link-based scheme, a traditional method, and the most congestion relief under either the link-based scheme or the MCP scheme. For the SAV scenario MCP appears to reduce VMT the most, while also significantly reducing delay. In order to benefit social welfare, revenue from any of the tolling schemes would need to be reinvested, particularly in transit or in improvements to pedestrian and bicycling facilities.
The technology required to implement some of the tolling schemes that connected vehicles make possible can be much cheaper than traditional tolling infrastructure, allowing a more widespread implementation.
Michael Brenneis is an Associate Researcher at SSTI.
Mileage-based fee more effective than California’s new ZEV fee
By Brian Lutenegger
California is among the states that have added a special registration fee for zero emission vehicles. However, research commissioned as part of the enacting legislation casts doubt on the efficacy of the fee in paying California’s infrastructure costs, instead pointing to a road user charge as the most effective solution.
California’s Road Repair and Accountability Act of 2017 (Senate Bill 1) added new taxes on gasoline and diesel fuel in that state to pay for roadway infrastructure. It also added an annual registration fee beginning in 2020 on new zero-emission vehicles (ZEVs), recognizing that these vehicles do not use gasoline for at least a portion of their operation. The new law also asked the University of California, Davis’ Institute of Transportation Studies to evaluate this new ZEV registration fee. That analysis was released in December.
While registration fees for ZEVs are not unique to California, that state has a higher adoption rate for this technology than others. So this new fee has the potential to have a greater impact in California than elsewhere, particularly if former Governor Jerry Brown’s goal of 5 million ZEVs on California roadways by 2030 is realized.
The UC-Davis researchers identify several major drawbacks of the registration fee. Assuming the expected shift toward ZEVs come to reality, they expect roadway infrastructure to be underfunded by more than $500 million annually. And while a gasoline tax is effectively based on usage, the ZEV fee is a flat fee regardless of how many miles per year the vehicle is driven.
Further, there are equity concerns. For example, plug-in hybrid owners would pay both gasoline taxes and the registration fee. And ZEV owners, the study authors found, would pay even more under the fee than with a gasoline tax alone.
Another critical issue is that the new fee may also suppress ZEV sales at a critical time in the development of the technology, preventing California from meeting its climate change and air quality goals.
The study concludes with a comparison of two other funding mechanisms: a tax on alternative fuels similar to the gasoline tax and a road user charge, i.e., a mileage-based fee. The researchers found that the road user charge applied only to ZEVs—leaving the gasoline tax in place for traditional vehicles—is the most sustainable solution. Although administrative costs for fee collection are a concern, the road user charge will ensure California can meet its funding needs and won’t be susceptible to reduced tax collection due to efficiency increases.
To recover declining gas tax revenues, at least 20 states have passed legislation requiring hybrid and/or plug-in electric vehicle owners to pay a special registration fee of up to $200 in addition to the standard fees.
While states need to recover lost revenue from declining gas tax revenues—due to both increasing fuel efficiencies as well as increased sales of ZEV and other hybrid vehicles—the study shows that a special registration fee is not the best way to do so. Such a fee will not allow California to pay its infrastructure costs and decouples the cost from actual vehicle usage. A road user charge is a far better way to ensure that drivers pay their fair share for their actual usage of California’s (and other states’) roadways.
Brian Lutenegger is a Program Associate at Smart Growth America.
Moving beyond LOS: The Pasadena story
The common practice of restricting infill development based on its impact on roadway level of service (LOS) has for decades depressed green, efficient compact development and induced energy-intensive, costly sprawl. Now many cities are moving to reform their land-use review process to lessen the emphasis on LOS, and instead to focus on system-wide impacts from development, as measured in vehicle-miles traveled (VMT). One of those cities – Pasadena, Calif. – pioneered the move away from LOS in 2015 and now has several years applying it to projects. (Spoiler alert: That experience is mostly good.) One of the heroes in that reform effort is Pasadena’s DOT director, Fred Dock. He joined us for a webinar that retraces Pasadena’s footsteps in developing the new policy and provides an evaluation based on three years of implementation.
Download the slides from the webinar.
