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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. !aplha_rtt.png|thumbnail! 
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