It's just simple maths - calculate the distance. Let's take the furthest distance - Tuas.
Should one have transports, it will take less than 20mins to reach there, or less than 2hrs in the case of traffic jams & also dependent upon how the authorities will manage the car park spaces as well as the cordoned area for Human space to rest & relax, with water, food, shelter & toilets .
If by walking, it may take 12 hrs at a non-sweating pace.
Such should only be done in times of emergency. We are a PREPARED ALWAYS Nation.