Accessing a random variable from an array for use in another varaible

I have one of four possibilities for this particular variable -->

var lowVelocity = Math.random() * (45 - 30) + 30;
var medVelocity = Math.random() * (60 - 45) + 45;
var highVelocity = Math.random() * (80 - 60) + 45;
var hwyVelcoity = Math.random() * (100 - 80) +80;

In one scenario I might have either lowVelocity and medVelocity so I want to choose randomly between those two only. So I did this by:

const velocities = ["lowVelocity", "medVelocity"];
const randomUrban = Math.floor(Math.random() * velocities.length);

And this works - it now chooses one of the two velocities that I am interested in. Low velocity is between 30 and 45 and med is between 45 and 60. In my innerHTML I want to print the number returned from that variable chosen in the array. When the user clicks this starts a function that has this in it.

document.getElementById("scenario").innerHTML = randomUrban; 

But when it prints in the HTML, it doesn't print the number that is associated with the variable, it prints the array number it chose (like 0 or 1).

How do I get the variable (eg if it chose lowVelocity then the number it prints will be the random number lowVelocity found, eg 39) to print instead of the array number?