Image via theroadswerenotbuiltforcars.com
Chat GPT Query: Use the “The cost-effectiveness of bike lanes in New York City,” Gu, Mohit, Muennig, Mailman School of Public Health, 8/10/2016, to model the cost effectiveness of the Grayways proposal as a public health intervention.
Chat GPT Response: Below is a model of the cost-effectiveness of the Grayways proposal as a public health intervention, using the 2016 Mailman study.
Baseline Study
The Study evaluated 45.5 miles of NYC bike lanes costing $8.1 million [$178K per mile] and found:
- 9.32% increase in cycling probability
- 2.2 QALYs gained per 1,000 residents over a lifetime
- ≈ 0.0022 QALYs per resident
- NYC population ≈ 8.8 million
- ≈ $1,297 per QALY (very cost-effective).
Results: Grayways cost per QALY
- Low-cost case ($5M): 5,000,000/21,300 ≈$235 per QALY
- High-cost case ($20M): 20,000,000/21,300 ≈$940 per QALY
Grayways Proposal
The Grayways proposal designates approximately 500 miles of weekend recreational cycling routes across New York City using existing low-stress streets, signage, and limited upgrades rather than full capital construction.
Because Grayways:
- Operates weekends only
- Is recreational rather than commute-substitution oriented
- Relies mostly on designation rather than protected lanes
We assume it achieves 10% of the per-mile behavioral effect estimated in the 2016 protected lane study.
📌 Step 1: Scaling Health Impact
From the 2016 study: 45.5 miles → 0.0022 QALYs per resident
Linear scaling to 500 miles: 500/45.5=10.99
Full linear effect (theoretical max): 0.0022×10.99≈0.0242 QALYs per resident
Applying the conservative 10% impact assumption:
0.0242×0.10=0.00242 QALYs per resident
Total citywide QALYs: 0.00242×8,800,000≈21,300 QALYs
So under conservative assumptions, Grayways would generate approximately:
≈ 21,000 lifetime QALYs
📌 Step 2: Cost Assumptions
Estimated total implementation cost range:
- Low case: $5 million
- High case: $20 million
Per-mile costs:
| Total Cost | Cost per Mile |
| $5M | $10,000/mile |
| $20M | $40,000/mile |
These costs reflect signage, wayfinding, traffic calming, and limited safety upgrades — not full protected lane construction reflected in the $178K per mile in the Mailman Study.
📌 Step 3: Cost per QALY
- Low-cost case ($5M): 5,000,000/21,300 ≈$235 per QALY
- High-cost case ($20M): 20,000,000/21,300 ≈$940 per QALY
📌 Interpretation
For context:
- Original 2016 protected lanes: ~$1,300/QALY
- Common public health cost-effectiveness thresholds: $50,000–$150,000/QALY
Even under heavily discounted behavioral impact assumptions, Grayways appears to be:
- Extremely cost-effective
- An order of magnitude more efficient than many public health interventions
- Comparable to or better than the original protected lane investment
📌 Important Caveats
This remains a modeled projection based on:
- Linear mileage scaling
- Assumed population exposure
- A 10% behavioral impact discount
- No adjustment for diminishing marginal returns
- No explicit modeling of crash risk changes
A full Markov simulation (as in the 2016 study) would refine these estimates further.
📌 Bottom Line
If Grayways achieves even a modest fraction of the per-mile behavioral effect of protected bike lanes, it likely delivers health benefits at a cost of hundreds—not thousands—of dollars per QALY, making it a highly cost-effective public health investment under standard economic evaluation frameworks.
