If your platform team only tracks "services onboarded" or "deployments per week," you are measuring compliance, not value. The single metric that predicts whether your Internal Developer Platform (IDP) will deliver return on investment is voluntary adoption rate of the golden path — the percentage of new work that chooses the paved road when an off-road option still exists. This article shows three ways to measure it concretely, using Backstage, GitHub, Argo CD, and Prometheus. It is the technical companion to the broader Platform Engineering business case — that piece argues why voluntary adoption matters; this one shows how to compute it. Why activity metrics mislead platform teams Most platform dashboards report on activity: Number of templates run Services in the catalog Pipeline executions per day Onboarded teams These numbers go up regardless of whether developers actually like the platform.…