on tipping points in traffic: just a few more cars

traffic density and flow chart

Say a typical freeway lane can handle approximately 2200 high-speed vehicles per hour at maximum capacity.

When 2400 cars per hour attempt to use that lane, congestion quickly reduces the average vehicle speed to a crawl and vehicle throughput falls accordingly to a few hundred vehicles per hour.

Yes, folks, it's that extra 200 cars that makes the difference.

Just so on Kauai roads.

For example, HI DOT says the 'capacity' of each lane running through Kapaa town is 125 vehicles per quarter hour.

In reality, there are many quarter-hours in the morning and afternoon when more than 150 vehicles are trying to squeeze through.

Just this extra 25 vehicles per quarter hour is enough to change traffic conditions from smooth flowing to bumper crawling.

Point being, much of our congestion is related to timing as is not a necessary consequence of growth.

Say wot? That's right, it's not so much that our traffic volume is growing rapidly, as that it's getting more bunched up.

BTW, accidents and other incidents have a similar effect as density increases.

In essence, this means we have a whole 'nother choice for helping solve our traffic mess. Stay out of Kapaa town during peak flows.

At 125 vehicle saturation flow rate, we can all move through Kapaa at 35 miles an hour. Or we can try to push 150 through and all sit there fuming.

This is especially true in peak visitor months, when that extra 10% of vehicles makes so much of our island less passable.

Now, if we could only get the visitors to kokua us by staying out of the high traffic areas at rush hour, too....

[Psssst...if you're interested in the physics of traffic and wot happens in these 'phase transitions' click here.]

Published by Ken on August 3rd, 2007 tagged Community Initiatives, Systems Thinking

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