I've been asked by email to publish some more material on driving variation out of the constraint. Specifically I was asked for software development and FDD examples but I'll get to those in a future post. First of all I'd like to look at one of Seattle's famous bottlenecks - the SR-520 freeway bridge across Lake Washington linking Seattle with its "east-side" suburbs of Bellevue, Kirkland and Redmond.
Anyone who works at Microsoft's Redmond campus and lives in the city of Seattle commutes across the lake on the SR-520. Equally suburban dwellers from the north east-side take it in the opposite direction to commute to Seattle. On an evening when there is a baseball game or a basketball game in the city, it can be jammed for 6 miles from the lake back to the Redmond campus. It's a notorious bottleneck every day morning or evening. So how would we drive variation out of this bottleneck?
The throughput of the SR-520 has a maximum of 60 cars per minute. That number is limited by the takt time (the time difference between moving cars) of 2 seconds. There are 2 lanes. So the maximum throughput is 30 cars per lane per minute. The 2 seconds comes from a well buffered "thinking time" for braking. It's the minimum safe time between vehicles. Empirical study of traffic has shown that maximum throughput is usually achieved at about 40 miles per hour. Faster speeds add extra variation to the system because not everyone can react fast enough and the result is that at faster speeds, a longer takt time between vehicles or greater variation in the gaps occurs. Below 40 miles per hour, maintaining a 2 second gap is difficult because the forward velocity is so slow. Below 20 miles per hour, it becomes impossible.
It's worth observing that takt time controls throughput and not velocity! Velocity affects lead time. At 40 miles per hour, the 4 miles of the narrowest part of the SR-520 on the east-side would take 6 minutes to navigate. Equally, the work-in-process inventory of cars on the freeway does not affect throughput directly. The goal to maximize throughput, therefore, is to find a way to maintain precisely 2 seconds between vehicles.
So we must look at what causes variation in the gaps between vehicles. Some causes will be - driver skill, attention span, eyesight, weather conditions, speed combined with reaction time of individual drivers, and lane switching. It's clearly hard for the traffic management people to control many of these but some they could affect. For example, lane switching. Why not simply ban it for the whole 4 mile stretch? Place double white lines between the lanes for the whole 4 mile stretch! This would have a significant effect. When the freeway becomes overloaded with too much inventory and speeds fall to a crawl, lane switching as people try to find the faster moving lane, actually causes things to get much worse. Eliminating lane switching is a way to drive variation out of the constraint.
What could be done to overcome some of the more human variations such as reaction time, eyesight and skill level? Well technology exists and has been tested by Mercedes on German autobahns to use ultrasonic or laser technology to measure the distance between vehicles. Such technology could be used like cruise control to maintain the 2 second gap between vehicles. Naturally, every vehicle using the bridge would need to be fitted with it. The combination of these two ideas would probably treble the throughput on the SR-520 at peak times. It's so compelling, it's a wonder that the employers on the east-side don't sponsor it ;-)
However, such technology is not full proof and accidents will still happen and an accident (or special cause variation) is devastating to throughput. What else could be tried to reduce the likelihood of a special cause variation?
Well ultimately, you take people out of their vehicles and put them in larger ones with a scheduled timetable running on rails. Anyone for a monorail extension across the lake?
