2026/03 – Reduction of CO2 resulting from Grayways

ChatGPT Query: [Use data estimating VMT reduction associated with proposed Grayways network to] Estimate carbon (“CO2”) emissions associated with this level of VMT reduction.

ChatGPT Reply: To estimate the carbon emissions associated with the VMT reductions you calculated for the Grayways proposal (~10.5 million to ~14.3 million vehicle-miles avoided per year), we can convert miles avoided into CO₂ emissions avoided using standard emission factors for U.S. passenger vehicles. 

Image Warren J. Wells

🔢 Emission Factor for Passenger Vehicles

According to the U.S. Environmental Protection Agency (EPA), the average passenger vehicle emits about 400 grams of CO₂ per mile driven (this reflects typical current U.S. fleet tailpipe emissions). 
That’s equivalent to roughly:

  • 0.4 kg CO₂ per mile
  • 0.0004 metric tons CO₂ per mile

📉 Annual CO₂ Emissions Avoided from Grayways VMT Reductions

Using the VMT reduction range from earlier:

Lower Estimate: 10.5 million miles avoided

  • 10,500,000 miles × 0.0004 tons CO₂/mile
    ≈ 4,200 metric tons CO₂ avoided per year

Higher Estimate: 14.3 million miles avoided

  • 14,300,000 miles × 0.0004 tons CO₂/mile
    ≈ 5,720 metric tons CO₂ avoided per year

Estimated carbon emissions avoided:
👉 ~4,200–5,700 metric tons of CO₂ per year

📌 Interpretation and Context

  • Per-mile emissions: The ~400 g CO₂/mile factor reflects a typical gasoline vehicle on U.S. roads today. 
  • This estimate only includes direct tailpipe CO₂ (not upstream emissions from fuel production or “well-to-wheels” aspects, which would raise the avoided emissions slightly).
  • Avoiding ~4,000–6,000 metric tons of CO₂ annually is roughly equivalent to taking about 900–1,200 average gasoline cars off the road for a year (based on ~4.6 metric tons of CO₂ per car annually). 

🧠 Notes on Scaling and Uncertainty

✔ If a significant share of avoided trips would otherwise be in more efficient vehicles (hybrids, higher-mpg cars, or electric vehicles on a low-carbon grid), the CO₂ avoided per mile would be lower.
✔ Conversely, if avoided trips are from less efficient vehicles (older cars, SUVs), emissions avoided could be higher than this estimate.
✔ This figure does not include reductions in other climate pollutants (methane, nitrous oxide), health-impairing air pollutants (NOₓ, PM), or co-benefits like reduced congestion.

📊 Summary

ScenarioVMT Avoided (annual)CO₂ Avoided (metric tons/year)
Lower~10,500,000 miles~4,200 tons CO₂
Higher~14,300,000 miles~5,720 tons CO₂

This provides a plausible carbon reduction range for the Grayways network based on reduced motor vehicle travel alone.