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The above graph shows how alpha varies with distance. Using a single alpha may fail for small RTTs (distances). This is probably since the min_RTT is dependent on router delays and where networks peer (so the routes may be very different from the Great Circle Distance (GCD) rather than global distance.
Looking at the graph above of Alpha vs min_RTT from SLAC it appears there are two regimes separated at ~8ms. Above 8ms alpha is ~ 50km/ms, below it is more variable closer to ~10km/ms
We also calculated median alpha from SLAC which is ~ 46.5km/ms and IQR ~15.6km/ms or IQR/Median~ 33% or ~ +-16%