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From the inbox:

I have another question for you regarding the aggregation of benefits after the individual demand curves are estimated.  My students will be using a Travel Cost/CV survey approach to estimate individual demand curves for six different resources ....  These include a bike trail, nature preserve, a children’s playground, recreational uses of the ... River, a skateboard park, and city park.  My question is: after the individual demand curve is estimated (using only personal interviews and not telephone surveys), what population number do you multiply by to get the aggregate demand curve?  It doesn’t quite make sense to simply use the local population of the surrounding area as not all folks avail themselves to these resources.  Any hints?

My answer:

You could use national recreation participation estimates from the National Survey of Recreation and the Environment to determine how many to aggregate over. Here is one link: http://www.srs.fs.fed.us/trends/Nsre/nsrerep.html.