Ryan Donahue of the Trust for Public Land says that it depends on age, health, time availability, quality of surroundings, safety, climate, and many other factors.
One problem with assessing how far people will walk to use a park is that most walkability studies are focused on commuters rather than park users who have a mix of motivations. However, it does seem that the quarter-mile standard used to assess the walkability of transit stations is mostly applicable to parks as well:
"...research supports the validity of both quarter-mile and half-mile distance goals, depending on perceptions of the built environment, safety, and time constraints. Of course, people's preferences and habits are only part of the equation for planners, who must also take into account the cost effectiveness of expanding the park system versus improving current parks or focusing on connectivity."
FULL STORY: Pedestrians and Park Planning: How Far Will People Walk?

National Parks Layoffs Will Cause Communities to Lose Billions
Thousands of essential park workers were laid off this week, just before the busy spring break season.

Retro-silient?: America’s First “Eco-burb,” The Woodlands Turns 50
A master-planned community north of Houston offers lessons on green infrastructure and resilient design, but falls short of its founder’s lofty affordability and walkability goals.

Delivering for America Plan Will Downgrade Mail Service in at Least 49.5 Percent of Zip Codes
Republican and Democrat lawmakers criticize the plan for its disproportionate negative impact on rural communities.

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Balancing Bombs and Butterflies: How the National Guard Protects a Rare Species
The National Guard at Fort Indiantown Gap uses GIS technology and land management strategies to balance military training with conservation efforts, ensuring the survival of the rare eastern regal fritillary butterfly.
Urban Design for Planners 1: Software Tools
This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.
Planning for Universal Design
Learn the tools for implementing Universal Design in planning regulations.
EMC Planning Group, Inc.
Planetizen
Planetizen
Mpact (formerly Rail~Volution)
Great Falls Development Authority, Inc.
HUDs Office of Policy Development and Research
NYU Wagner Graduate School of Public Service