Question:  Freeway access to Valley Fair Mall in San Jose is so bad I never shop there.  Why can’t someone build new ramps from northbound I-280 to Winchester Boulevard? 

Answer:  The brief answer is that the on-ramps to northbound I-280 from I-880 would be too close to these new off-ramps and create a dangerous weave pattern in the traffic.  Note that in the diagram below the traffic streams from A to D and from B to C have to cross within the distance labeled as the Weaving Area.  As long as this distance is long enough, the weave area operates safely and efficiently.  If it is too short, traffic can slow down drastically. 

Caltrans traffic operations staff have reviewed hundreds of these types of interchanges over the last few decades and developed a couple of important policies about the spacing of freeway interchanges:

  1. Interchanges must be a minimum of 1 mile apart so the on-ramp traffic from one interchange will not interfere with the off-ramp traffic from the next. 
  2. When one of the interchanges is a major freeway-to-freeway interchange, the spacing increases to two miles. 

See http://www.dot.ca.gov/hq/oppd/hdm/pdf/english/chp0500.pdf for a more detailed discussion.  These general spacing guidelines provide a reasonable assurance that the weaving volumes will function safely and efficiently. 

In locations where the appropriate spacing can not be provided, a ramp configuration called a braided ramp can be installed.  A nearby Santa Clara County example is the interchange of I-880 with Great Mall Parkway in Milpitas (see photo).  In this case, the traffic stream from B to C travels on a raised structure over the ramp from A to D, so there is no at-grade (aka – at the same level) weaving movement.  Braided ramps are expensive, though, and don’t get built if there is no funding source. 

This is also a great question for Mr, Roadshow, Gary Richards, who would likely be able to provide a specific history of the area.  You can find him in the San Jose Mercury news or at http://www.mercurynews.com/mr-roadshow, and you can send a question to him at grichards@mercurynews.com