This is a follow-up to Can I Ready an action to Disengage?
It looks to me that the 5E RAW does not have a simple way to declare a behaviour like this:
If anything nasty comes towards me, I run in the opposite direction!
That is due to how the Dash action is worded as an increase in movement allocation for your turn, and not as an additional segment of movement in its own right:
When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash.
So, importantly, Dash does not make you move, instead it adds to your "movement allowance" for the turn.
In addition the movement rules say nothing about movement except during your own turn (emphasis mine):
On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.
There are no rules about having or using movement on other turns than your own. Is this an oversight?