If it\’s got an engine. . .

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To merge or not to merge

Posted by ifitsgotanengine on September 25, 2005


The next time you are driving down the interstate, minding your own business, and you see a sign “lane ends 2000 feet”, ask yourself “Why?”.

If the traffic count was high enough before the change to justify three lanes, and there was no off-ramp, where did the cars go?

That’s right; nowhere.


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There is never a good reason to change the number of lanes without an associated on-ramp or off-ramp.

This is important. It means that we are spending millions of dollars unnecessarily. Consider the situation where two lanes expand into three. This gives no extra capacity, as the total throughput is still limited by the bottleneck; that is, the total capacity is the same as if the entire distance had been two lanes.

The converse is true as well. If three lanes merge into two, there is no added benefit in terms of throughput of the portion that was three lanes.

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4 Responses to “To merge or not to merge”

  1. Anonymous said

    Years of analysis would provide the traffic engineering folks the true measure of their original prowess. If the section of road experiences a disproportionate number of accidents or delays, the engineers would either have to admit they made a mistake, or try to put a band-aid on their clogged artery. Call it “asphalt angioplasty”.

  2. Dorri said

    Thanks Chris.

  3. Anonymous said

    You’re quite welcome.

  4. Anonymous said

    um… you wouldn’t necessarily need to add an additional off-ramp if the existing number of off-ramps (plural) were not yet bottlenecks, right?

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