Climate Resilience Dashboard
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In April 2021 the TRPA Governing Board updated AQ14. The update replaced the 40 year old NOx based standard (AQ14), with a standard grounded in the Region’s transportation and land-use goals and the vision for a more walkable, bikeable, transit-oriented, sustainable Tahoe, laid out in the Regional Plan and Regional Transportation Plan. Additional detail can be found in the staff report.
Threshold standard (AQ14) was established in 1982, and set a goal of reducing in-basin nitrogen emissions by 10% from 1981 levels, and benchmarked its performance to total regional VMT. In 1981 increased algal growth because of elevated nutrient inputs (phosphorus and nitrogen) was thought to be the primary driver of Lake Tahoe's clarity loss. Since 1982 a number of developments have occurred that have functionally rendered the original intent of the nitrate reduction threshold standard (AQ14) moot. First, improvements in tailpipe emissions controls have reduced nitrogen emissions by more than 66%, far greater than the 10% objective of the adopted standard, functionally accomplishing the goal of the standard. Second, the Total Maximum Daily Load (TMDL) scientific research established that fine particles were the principal driver of clarity loss. Each of the last four threshold evaluations have recommended that the 1982 VMT nitrogen deposition threshold standard (AQ14) be reviewed and updated.
The Governing Board is considering an update to the standard, that would relaign the standard from one rooted in concerns over nitrogen emissions to a standard that estimates a goals for reducing reliance on the automobile, reducing GHG emissions, and promoting mobility.
- Status
- Insufficient Data to Determine Status or No Target Established
- Trend
- Insufficient Data to Determine Trend
- Confidence
- Low
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The goal of the standard, a 10% reduction of mobile source NOx emissions from 1981 levels was accomplished more than 25 years ago. Mobile source NOx emissions today are less than a third of what they were in 2000 and are forecast to continue to decline as a result of increasingly clean automobiles.
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While the intent of the standard was reducing NOx emissions, and four consecutive threshold evaluations have suggested the NOx - VMT relationship should be revisited, the standard has typically been evaluated as it is written, through VMT. VMT in 2018 was estimated to be 1,393,994, 3.4% lower than it was 1981 (1,443,319). The 3.4% drop in VMT is short of the 10% reduction identified in the standard.
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NOx emissions from mobile sources have rapidly declined over the last 20 years.
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VMT in Tahoe has remained within a relatively narrow band since the standard was adopted in 1982. VMT has generally fluctuated with macro-economic conditions, but never increased or decreased more than 10% 1981 levels.
- This indicator measures VMT as a proxy NOx emissions from mobile sources.
- The relationship between VMT and NOx emissions has changed significantly over the last 40 years as a result of increasingly stringent tailpipe emissions standards. Nationally, VMT continues to increase while NOx emissions have drastically declined. Nationally, NOx emissions have decreased by 57 percent since 1980 despite a 49 percent increase in VMT since 1990 (TSAC 2018a). Locally these changes mean that a 14-fold increase in VMT from 1981 levels would be required to generate 1981 NOx emissions levels. Vehicle miles traveled (VMT) is a measure of the number of miles driven on roadways in a specified area and period of time. Estimates of VMT are generally approximations of actual vehicle miles traveled, based on estimates of trip distance and frequency (Salon et al. 2012). VMT could be precisely measured using car odometers, but rarely is because of the difficulty in obtaining the information (Salon et al. 2012) and the challenge of determining where the vehicle travel occurred. VMT is influenced by a complex set of interconnected factors and synergies between individual factors. For example, higher fuel prices reduce regional VMT, but the response at the household level is influenced by household location and income (Salon et al. 2012, 2013). Nationally, VMT has generally increased as the population has grown, the economy has expanded, and car ownership has increased. The Federal Highway Administration (FHWA) forecasts suggest that nationwide VMT will continue to grow by 1.07 percent annually through 2035. The FHWA forecast is influenced by projections for population growth, economic growth, and increased disposable income, all of which are positively associated with VMT (FHWA 2017). VMT in the Tahoe Region is a function of the complex interplay of a variety of factors including population (both inside and outside the Region), gas prices, employment rates, local housing costs, demand and access for recreational opportunities in the Region, and access to alternative forms of transportation. Higher unemployment, higher fuel prices, increased congestion, work from home programs, employer car pool programs, and concentration of development in centers are all linked to reductions in VMT. While population growth, higher household income, higher employment rates, increased fuel economy and greater roadway capacity are all linked to increasing VMT. Increasing access to transit services, access to bicycle and pedestrian facilities, and the relative desirability of alternative modes of transportation in comparison to the use of the personal automobile can reduce VMT.
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Monitoring Programs
- The goal of the standard, a 10% reduction of mobile source NOx emissions from 1981 levels was accomplished more than 25 years ago. Mobile source NOx emissions today are less than a third of what they in 2000, and are forecast to continue to decline as result of increasingly clean automobiles.
While the standards intent was reducing NOx emissions, and four consecutive threshold evaluations have suggested the NOx - VMT relationship should be revisited, the standard has typically been evaluated as it is written, through VMT. VMT in 2019 was 4% lower in 2019 than it was in 1981. The 4% drop in VMT is short of the 10% reduction identified in the standard. - NOx emissions from mobile sources have rapidly declined over the last 20 years.
VMT in Tahoe has remained within a relatively narrow band since the standard was adopted in 1982. VMT has generally fluctuated with macro economic conditions, but never increased or decreased more than 10% 1981 levels.
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