Travel Time Platform is a website that lets users draw Isochrones, areas which can be reached in a given amount of time (Iso from the Greek for same, chronos for time). I have used it to draw a time radius. Here we show a 30 minute walking time from the Seymour Centre (near the WalkSydney world headquarters, but you can choose anywhere.)
The 30 minute city is a concept about accessibility, can the important places travelers want to go be reached in a given time. The idea that 70% of the people can reach daily activities within 30 minutes of walk, bike, or transit is embedded in the most recent Metropolis of Three Cities plan of the Greater Sydney Commission.
A thirty minute walk gets you a reasonable distance, but this is hardly a circle, the network is not as direct as it might be. Notice in particular the warping at the base. The issue here is that the railway tracks through Darlington have no crossing between Redfern and Erskineville. So instead of being able to get to Beaconsfield in 30 minutes, I am stuck in Alexandria. The logical shape of an isochrone on a perfectly radial network from the center is a circle, and on a perfectly grid network is a diamond. While closer to a circle than a diamond, the bottom of the circle is smooshed due to the lack of cross-track connectivity. In addition to railway tracks, bodies of water and motorways distort the network and reduce pedestrian accessibility.
The picture looks different by bike (assuming every link is bikeable — which is not true for most people), car, and transit of course, shown below. The car provides the greatest access (hence explaining its use), and cycling beats transit, but if we want people to walk, taking a small amount of time from the car would make a big difference to people on foot.