Why, on part of I-5, do the N and S bound lanes cross over each other, so you drive on the left?

Question

In NW L.A. county, there is a section of I-5 where the traffic in one direction crosses over or under the traffic in the other direction, so that you drive on the left side of the oncoming traffic (which is far away, on the other side of a canyon) for a while, until you again cross over or under the uncoming traffic, and return to the right side.

Why?

(If you do not believe me, go to http://www.mapquest.com/maps/map.adp?formtype=address&country=US&popflag=0&latitude=&longitude=&name=&phone=&level=&addtohistory=&cat=&address=royal+rd+and+the+old+rd&city=castaic&state=ca&zipcode=.)

This is between Castaic and exit 183.

Traveler: I understand why the Northbound and Southbound lanes are not all on the same side of the canyon. My question is why the northbound lanes are on the west side and the southbound lanes are on the east side, like one would expect in the U.K., rather than the northbound lanes on the east side and the southbound lanes on the west side, like other roads in the U.S.

Answer 1

I know exactly where you are talking about and you have partially answered your own question. There was an old freeway that ran through the canyon and when the built an enlarged freeway system through the area there just wasn't enough space due to terrain considerations. It was just easier to route the road along both sides of the the canyon than to expand the existing bridges and abutments. It was cheaper and safer to build new and separate.

I do understand. Terrain considerations are the reason. yes they do cross over and the reason is terrain. A more gradual rise on the north bound lanes.

Answer 2

The person before me answered it very well, and the only thing I can add is that the grading and steepness of the two sides are significantly different, and it just happens that the opposite sides favor the uphill climb and the downhill glide. Secondly, I think they would have had to build multiple tunnels and/or earthworks to build it the normal way.

Answer 3

As you enter or leave the Grapevine the lanes do cross. That is because of the way the mountains are. The architect had to do that to bypass the dips and valleys.