As the United States moves toward a fleet comprised of more EV and hybrid vehicles, important questions impacting the design and management of critical assets must be considered in future practice and policy making. For instance:
- Are heavier EV’s and hybrid vehicles deteriorating our bridges, pavements, and other assets before the end of their design/service lives due to their heavy loads?
- Will the live load increases to bridges and axle weight distribution from heavier EVs and hybrids impact asset lifecycle needs and level of service performance?
- Are new design standards needed for the engineering design of transit and freight roadways and bridges to reflect the increased live load changes?
- How is the accelerated rate of deterioration impacting asset lifecycle needs?
Answering these questions is critical as the types of vehicles using state and local transportation facilities transition to the next generation of vehicles.
Since asset management (AM) is a business process and decision-making framework using economic and engineering modeling over an extended time horizon, it can help inform many aspects of designing and planning for and implementing the expected service levels of roadways and structures to support adoption of next-generation vehicles. Developing guidance that reflects changes associated with EV and hybrid vehicles will lead to updated design models, more reflective deterioration rates, and improved planning for the preservation, improvement, and operation of road and bridge assets while protecting them from shorter lifecycles.
Single occupancy vehicles, while a major cause for congestion, do not appear to provide measurable impacts to load weighting. Therefore, the next-generation vehicles considered under this study will include a comprehensive mix, including transit, garbage trucks, and freight. For bridge analysis, this research would evaluate existing data and account for the battery weight to develop a design load model for gross vehicle weight. Pavement analysis utilizes equivalent single axle load (ESAL) ratings.
The figure linked below, provided by the City of Seattle, illustrates the tremendous load impacts that buses and trucks have on pavements and bridges:
The City estimated that it would take about:
- 1,500 cars to equal the damage of the typical 18-wheeler on a Washington State Highway
- 2,500 cars to equal the damage of the average empty bus
- 5,000 cars to equal the damage of the average full bus
- 8,000 cars to equal the damage of an average full 60’ Articulated Hybrid
WSDOT, in its pavement design guide, advises that the average 18-wheel, double unit truck on a state highway has a total ESAL factor of 1.00 to 1.35 on average, a figure reached through their different studies of truck weights (at weigh stations, etc.). As the picture shows, the design loading is significantly lower than those being applied by buses, especially those with heavy batteries.
Transit buses have a federal exemption from axle weight limits that dates to the ISTEA transportation legislation in the early 1990s. Buses have gotten increasingly heavy over the last two decades with new equipment like hybrid powertrains, yet the number of axles remains the same because of the exemption. Furthermore, transit agencies are often not the owners of the roadways and bridges so they may not consider infrastructure conditions and design into their decision making.
The average bus empty is more damaging than the typical truck on a state highway. Add passengers, the exponential relationship between axle weight and fatigue, and the damage factor per bus increases to almost six times for the heavy articulated hybrids.
The figure hints at why agencies are struggling to maintain roads and bridges. As the transportation system is expected to carry heavier vehicles than they were designed for, it is critical to update design models and develop new deterioration factors to be used in asset lifecycle planning.
In addition to design and maintenance issues, the study will also consider environmental and social impacts and potential risks associated with next-generation vehicles. The inclusion of these factors enables asset management cross-asset trade-offs to be evaluated in terms of both positive and negative impacts. A comprehensive, triple bottom line study will pull from existing sustainability and economic lifecycle cost analyses on EVs and infrastructure construction along with societal factors such as increases in travel time from driving on deteriorated infrastructure assets.
The objective of this study is to develop a guidebook with suggestions for updating asset management models used in pavement and bridge design and preservation. The guide will be developed from research to determine:
- The impacts of EVs and hybrid vehicles on typical pavement and bridge (component and element) deterioration rates and their potential impact on lifecycle planning strategies.
- Environmental and societal impacts associated with the use of EVs and hybrid vehicles to support a triple-bottom line analysis and cross-asset tradeoff assessments.
- Suggested modifications to pavement and bridge load models used in design activities.
- The feasibility of establishing battery weight limits, increased road user fees, or other strategies for addressing asset management impacts.
The guidebook will be supported by case studies from a variety of agencies (state and local, urban, and rural) illustrating the potential impact the resulting deterioration rates would have on pavement and bridge designs, maintenance strategies, and lifecycle costs.