Strava has introduced a new metric for its premium users called Relative Effort. The new feature replaces Strava's Suffer Score as a new heart-rate based score that aims to assign a robust and consistent value across all your sporting activities, allowing you compare efforts.
This actually reminds us of the hrTSS score used by Training Peaks, another activity tracker that uses a heart rate based metric to calculate exercise intensity.
However, it has been shown that power-meter based measurements are generally more consistent for gauging training intensity. That said, power meters are out of reach for many cyclists and, as such, having some sort of metric is definitely of value.
Strava has leveraged its huge user base to gather data for analysis on heart rate data in an effort to make the Relative Effort score consistent, regardless of the type of effort or activity. You can read more about the project in Strava's article Quantifying Effort through Heart Rate Data on Medium.
Strava attempts to normalise the Relative Effort score in order to “level the playing field,” allowing users to compare scores not just between activities and sports, but also different users.
It also attempts to indicate your training volume, allowing you to stay on track for recovery, maintain fitness or warning you before you start overtraining.
The training tools that Strava provides remain relatively superficial and give less insight than those provided by more analytical programs such as TrainingPeaks. But, as a rough gauge of your training load, Relative Effort should be quite useful as an indicator of effort.