This map shows every transit stop you could reach within 30 minutes, by walking and using King County Metro-operated transit service, if you were to start at the Beacon Hill Link Light Rail Station on a typical weekday. This is determined by generating a collection of reachable stops at each time of day, starting at midnight and repeating every minute thereafter until 23:59 (hover over each point for the number of times out of a total 1440 times). The hue of the dot shows how many times of day you could leave Beacon Hill Station and reach the dot within the allocated 30 minutes (ROYGBV ordering; bright red signifying least, dark violet most). Thus, you can use this map to determine which destinations you could plan on reaching if you just happened to be at Beacon Hill Station at some point in the day and which ones you would want to do more explicit planning to reach. For example, there are only two times of day where you could make it the Northgate Mall within 30 minutes, so you'd need to plan that trip more carefully than a trip to the Triple Door, which can be reached within 30 minutes 86% of the day.
This map has several limitations. The map does not include service operated by Community Transit and Pierce County Transit on behalf of Sound Transit, nor does it include the Monorail. It only shows which transit stops are reachable, whereas it would be more useful to show all points that can be reached (i.e. the walkshed from a stop given the duration of time remaining in the trip when arriving at that stop). Transit times are taken directly from King County Metro's General Transit Feed Specification files and may not represent a realistic schedule given traffic. Walking times are are generated using Google's Distance Matrix API and may not be representative of all transit riders.
I intend to (slowly) improve the software that generates these maps to address these limitations and eventually allow anyone on the internet to generate maps like these with a configurable set of parameters. I am hoping isochrone maps like this one can be used by those who care about about transit—whether personally or professionally—to make more compelling and illustrative arguments when evaluating or proposing modifications to the transit network that they use.