Starting April 1st, 2021, Pantheon will start enforcing their previously-documented site traffic limits; see Pantheon Hosting Rate Increases. This means that if your site receives more traffic than is allowed within your hosting plan, you may be upgraded to a higher plan.
Pantheon Site Traffic vs. Other Analytics (e.g., Google Analytics)
Pantheon's "Visits" is what they are using for determining the appropriate plan, and it is closest to “Pageviews” in Google Analytics; however, Pantheon has an FAQ on "Why doesn't Pantheon's traffic metrics match my other analytics?":
Pantheon tracks every single request to the platform. In contrast, analytics tools will typically only track complete "pageviews" on an HTML page containing a tracking snippet that can fire off a subsequent request to the analytics platform.
See the above FAQ for more details. Pantheon's metrics track visits from sources that Google Analytics does not typically track; e.g., API requests, automated traffic from bots, and when users close the browser before the tracking script loads.
If you have questions about the specific metrics for your Pantheon site, please contact Pantheon support and request an audit for your site. If your site is upgraded to a higher plan due to exceeding Pantheon's traffic limits, you may downgrade your hosting plan at a later date if your site's traffic goes back down for two consecutive months. If this happens, please submit the Change Hosting Plan form.
How to Check Your Pantheon Site Traffic
Pantheon's site traffic information can be found on the "Live" tab of your Pantheon site dashboard. Go to your Pantheon site dashboard, and navigate to the "Live" tab, then select the "Metrics" tab. The default view in this section will show you daily metrics; select the "Week" or "Month" tabs to see weekly and monthly traffic. See Figure 1, below.
Figure 1: Select "Metrics" item on the Live tab of Pantheon site dashboard