I am currently planning an accessibility analysis for the recycling infrastructure in my city (waste collecting points) and I want to determine the walking time from each point so that I could seek for the areas which are not covered by the infrastructure.
Using a questionnaire, I've found out that the people in my research area usually walk up to 20 minutes in order to reach their collecting points, so I would like to build the "buffers" for 4 time intervals (less than 5 minutes, 5 to 10, 10 to 15 and 15 to 20 minutes).
What should I do in order to accomplish this task?
I seek any help (including tutorials, videos, reading materials and so on).
Below is an example of what I'm trying to get (the exemplified map shows the walking time from supermarkets)