I could not find anywhere in the default stroke options, so I played a little myself, it is very close to the default, I think:
directionsDisplay = new google.maps.DirectionsRenderer({
polylineOptions: {
strokeColor: '#0088FF',
strokeWeight: 6,
strokeOpacity: 0.6
}
});
DEMO Fiddle Google: http://jsfiddle.net/z9hJd/