javascript - Populate one dropdown based on selection in another then redirect

I'm trying to build a two tier drop down menu where the
second drop down populates the second. I've found a lot of examples on the site but I
want my menu to redirect to a page after the second menu is selected and can't figure
that bit out.

I'm not that up to speed with JS
so please bear with me.

The code
below is an example from another
post:

            type="text/javascript">
function configureDropDownLists(ddl1,ddl2)
            {
 var colours = new Array('Black', 'White', 'Blue');
 var shapes =
            new Array('Square', 'Circle', 'Triangle');
 var names = new Array('John',
            'David', 'Sarah');

 switch (ddl1.value) {
 case
            'Colours':

 document.getElementById(ddl2).options.length =
            0;
 for (i = 0; i < colours.length; i++) {

            createOption(document.getElementById(ddl2), colours[i], colours[i]);

            }
 break;
 case 'Shapes':

            document.getElementById(ddl2).options.length = 0; 
 for (i = 0; i <
            colours.length; i++) {
 createOption(document.getElementById(ddl2), shapes[i],
            shapes[i]);
 }

 break;
 case
            'Names':
 document.getElementById(ddl2).options.length = 0;
 for (i
            = 0; i < colours.length; i++) {

            createOption(document.getElementById(ddl2), names[i], names[i]);
 }

            break;
 default:
 document.getElementById(ddl2).options.length =
            0;
 break;


            }

}

function createOption(ddl, text, value)
            {
 var opt = document.createElement('option');
 opt.value =
            value;
 opt.text = text;

            ddl.options.add(opt);
}

Then
call it

            onchange="configureDropDownLists(this,'ddl2')">
            value="">
            value="Colours">Colours
            value="Shapes">Shapes

            value="Names">Names

            id="ddl2">

This
all works fine, but I want the page to redirect to somewhere on the site after the
person makes a selection in the second drop
down.

Could anyone help with how to adapt the
code to make that happen?

Thank
you