[Animats]

The Saudis have tried to fix the problem, but not very effectively. They've built lots of tunnels and ramps. But they tend to build hard-sided walkways without escape routes. They also have too many long pathways where people can't see what's happening ahead. So anything that causes a stoppage can cause a crush.

The Mina crush may have been started by a VIP visit by the Saudi minister of defense, with a big entourage, interfering with the traffic flow. The 1990 tunnel crush was triggered by some people falling off a bridge over the tunnel exit, stopping traffic at the exit to a long tunnel.

But it's the long, inescapable walkways which are the problem. There's a good analysis at a crowd safety site.[1]

A German firm did the crowd planning for the Grand Mosque capacity upgrade in 2010, and that seems to be working well.[2] That's six miles from Mina. The Grand Mosque area is now insanely overbuilt (the giant five star hotel with giant clock tower, run by Fairmont Hotels, across from the Kaaba, looks like something from Vegas) but has much more traffic capacity.

[1] http://www.workingwithcrowds.com/2015/09/hajj-pilgrimage-mor... [2] http://spectrum.ieee.org/tech-talk/computing/software/hajj-p...

[tantalor]

Sounds like a blood clot.