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?

Planetizen Federal Action Tracker
A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

Canada vs. Kamala: Whose Liberal Housing Platform Comes Out on Top?
As Canada votes for a new Prime Minister, what can America learn from the leading liberal candidate of its neighbor to the north?

The Five Most-Changed American Cities
A ranking of population change, home values, and jobs highlights the nation’s most dynamic and most stagnant regions.

San Diego Adopts First Mobility Master Plan
The plan provides a comprehensive framework for making San Diego’s transportation network more multimodal, accessible, and sustainable.

Housing, Supportive Service Providers Brace for Federal Cuts
Organizations that provide housing assistance are tightening their purse strings and making plans for maintaining operations if federal funding dries up.

Op-Ed: Why an Effective Passenger Rail Network Needs Government Involvement
An outdated rail network that privileges freight won’t be fixed by privatizing Amtrak.
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.
New York City School Construction Authority
Village of Glen Ellyn
Central Transportation Planning Staff/Boston Region MPO
Institute for Housing and Urban Development Studies (IHS)
City of Grandview
Harvard GSD Executive Education
Regional Transportation Commission of Southern Nevada
Toledo-Lucas County Plan Commissions