90-Second Headways

The elusive 90-second headway. Like a unicorn, or Sasquatch, or some other mythical beast, the promise of waiting no more than a minute and a half for a train during rush hour is what keeps some us going.

But apparently, this wouldn’t be good for service. A study conducted in 2006 by SYSTRA for WMATA used a proprietary software tool called the RAILSIM network simulator, which sounds like fun and I’d like to get my hands on it. But most intriguing were their findings on ATO and higher headways:

SYSTRA also investigated the feasibility of operating trains at 90-second headways on the Metrorail system using RAILSIM Network Simulator and a modified version of the database previously developed for the Metrorail network. SYSTRA evaluated the simulated running times to develop minimum supportable headways and sustainable train separations for each line in each direction. The analysis results show that the Metrorail system can operate on 90-second headways at most system locations when necessary to achieve schedule recovery after a delay has been incurred. However, 90-second headways offer no schedule recovery capability and should not normally be reflected in a scheduled operating plan.

I think essentially what the last point means is that if you’re running 90-second headways and you get an offload, or whatever other issues might will arise, it’s hard to run enough replacement trains to pick up the slack. Which seems like a valid point. But of course, if we were running 90-second headways – or even 3 minute headways – the crowding issues that offloads result in now would be minimized.

Still, at any rate, it’s nice to see that dreams are possible – even if they haven’t yet come true.


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