Every policy announcement moved the mobility line down a step

Community mitigation — personal protective measures, social distancing,
surface cleaning — is what is available **when medical countermeasures such as
vaccines or therapeutics are not**, which in March 2020 was everything.

The open question was whether policy did anything, or whether people were
already changing their behaviour on their own.

What was measured

26 February – 1 April 2020, in the metropolitan areas of **San Francisco,
Seattle, New Orleans and New York City**: the timing of **public policy
measures**, against community mobility — a proxy for social distancing —
and reported COVID-19 case counts.

**Community mobility declined, decreasing with each community mitigation
policy issued and as case counts increased.**

Two things moved it: the policies, and the news. Mobility fell in steps as each
measure was announced, and it also fell as cases rose — so voluntary behaviour
change was real, and policy added to it.

That distinction matters. **Voluntary adoption of social distancing by the
public and community organisations is possible**, and the data says **public
policy can enhance implementation** — which is the claim the stepped decline
supports.

The limit, stated plainly

**Public policies to increase compliance with community mitigation strategies
might be effective in decreasing community mobility; however, more
information is needed to assess impact on disease transmission.**

Mobility is not transmission. This report establishes that policy changed
behaviour, and explicitly declines to claim it changed the epidemic — because
in early April 2020 the case data could not yet show that, and saying so would
have been guessing.

That restraint is what makes the first finding usable: a measured,
short-latency indicator that tells you whether a policy is landing, weeks
before the case curve could.

The framework it tests

**CDC's Community Mitigation Framework recommends a phased approach at the
community level**, escalating as evidence of community spread increases and
easing as it decreases, scaled to severity. This is four cities' worth of
evidence on the first half of that: when the phases were announced, movement
fell.

Source: Centers for Disease Control and Prevention, MMWR.