Last week, I ran a study for a client whose subdivision is having a trailhead installed adjacent to them.  Residents are concerned about property values, traffic, aesthetics and crime.  Since this is becoming more common around St Louis city and county, I thought it was worth sharing.  
 
My study points were 6 parking lots serving as trailheads along the Grant's Trail in South County running from Crestwood to Hwy 55, about every mile.   For each of the 6 locations, I ran two sets of 3 statistics, a 1/4 mile circle and a 1 mile circle, with the 3 statistics being average sale price, days-on-market, and list price to sale price ratio.  The small set (1/4 mile) focuses on any negative effects the parking, traffic, noise and crime, if any, may have vs. the positive effects being close access to the trail and greenspace, may have on the desirability of homes in the immediate area.  Then compare that to a larger area (1 mile) surrounding it.  This keeps as many factors as possible the same - age and style of home, school district, seasonality, etc, and attempts to find a pattern by looking at 6 different points along the trail.  Of course, with the relative small sample sizes and other factors affecting the outcomes, this study is far from scientific.  However, I believe it is valuable.  I also spoke to a couple residents for their thoughts, and looked up prior studies around the country (found quite a few) that have paid for similar research.  A home right next to the trailhead that may experience overflow parking and more commotion would be more affected than a home 1 or 2 blocks away, which this study does not discern.
 
What I found is this:
 
3 of the 1/4 mile groups (neighborhoods closer to the parking lot/ trailhead) were higher average sale price than the larger, surrounding areas, and 3 of the 1/4 mile groups were lower.  Comparing all 6 of the differences, the average price (judging by percent difference to control variances, not by actual price) was 3.7% higher in the 1/4 mile groups.  Judging by actual prices came out 7.5% higher in the 1/4 mile groups.  Do not take this as interpreting that you should expect 3.7% higher sale price if you live within 1/4 mile of a trailhead, (and certainly not 7.5%) as I would need to control my comparisons much more to be somewhat more reliable.  However, with my brief study, and my understanding of statistics and percentages, that is the figure that came out at the end.
 
To go a step further, 4 of the 1/4 mile - closer neighborhoods - sold quicker than the larger surrounding areas, and 2 took longer to sell, as evidenced by days-on-market.  4 of the 1/4 mile neighborhoods sold for closer to list price, or even above list price, than the surrounding areas, while 2 came off their list price more, as evidenced by sale price to list price ratios.  These stats support the slightly higher average sale price found in the 1/4 mile groups.
 
I believe this all goes to support the argument that was presented almost universally in the other studies I found around the country, that being close to a trailhead and/or parking lot does not hurt your property value, and in fact may enhance it.  However, I would caution that if you are right next to it, I do expect a negative effect due to aesthetics of structures, cars, and commotion.  I would not anticipate any crime increase, although a phone call to your local police department may give you a better answer on that.
 
Happy Trails!