The Ventura Freeway, running from Woodland Hills to Pasadena, is the Valley’s main – only, really, since the 118 is so far north – east-west freeway. In typical LA fashion, much of the Valley is car-oriented despite the paucity of freeway capacity: just one east-west freeway for nearly 2 million people. The freeway is the famous 101 from Woodland Hills to the Hollywood Split, and the much less famous 134 from the Hollywood Split to Pasadena.
The Ventura Freeway is newer than some of LA’s first freeways, like the 110, the 10, and the Hollywood Freeway portion of the 101, so it doesn’t suffer from the problem of short interchange spacing. However, it’s old enough to have underpowered freeway-to-freeway interchanges at the 405, the Hollywood Split, and the 5. Let’s take a closer look at these three interchanges, as usual with an eye on rationalizing the freeway facility and improving the local streets in the vicinity. For this post, we’ll look at the Hollywood Split, saving the 405 and the 5 for another day.
Before diving in, let’s think about what makes a good freeway-to-freeway interchange. First, these interchanges have larger ramp volumes than typical interchanges, which makes the weaving conflicts worse. That suggests an increased need to avoid conflicting local ramps (a local on-ramp right before the freeway off-ramp, or a local off-ramp right after the freeway on-ramp). Second, these interchanges take up more space than typical interchanges. Therefore, in order to maintain functionality for local traffic distribution, it often makes sense to have an interchange serving local traffic integrated somehow. This can take the form of half diamonds or a full diamond interchanged arranged so that its ramps don’t cause any weaving conflicts. I like to call this an “inside interchange”. Here’s an example: the 134 and the 2, with inside diamonds on the 134 at Harvey and the 2 at Holly.
Now, we don’t want to go blowing massive holes in North Hollywood to drop in 65 mph ramps and create interchanges like they have in undeveloped parts of Fontana. However, these principles can still help us figure out what will work for these cases. We’ll do our best to keep improvements within available right-of-way, compromising on speed where needed. Alright, on we go.
The Hollywood Split is the somewhat confusing junction of the 101, the 170, and 134 in North Hollywood. The 101 enters from the southeast on the Hollywood Freeway and departs to the west on the Ventura Freeway. The leg to the northwest is the 170, and the leg to the east is the 134, both of which end at the interchange.
Despite being signed as the mainline freeway, the 101 exits on the right and merges on the right going east/south, and exits on the right and merges on the left going north/west. This is contrary to modern design standards, which require that the mainline freeway stay left, with exits and entrances to the right.
On the other hand, the interchange also reflects downtown-oriented design, with the movements to/from downtown emphasized at the expense of other movements. This reflects the thinking of the era, that people would drive to a downtown central business district (CBD) in the morning and out to suburbs in the afternoon. With LA’s polycentric development, downtown is not as dominant as it is in many cities. The Ventura Freeway provides important east-west connectivity to outlying CBDs in Sherman Oaks, Burbank, Glendale, and Pasadena. Thus, if one considers the Ventura Freeway the mainline – an argument for which there is a good case, as we shall see – there are only two through lanes, which is also substandard for this location. The Ventura Freeway through movements stay left at the splits entering the interchange, but merge to the right departing the interchange. This means that depending on which movements dominate, we might make different decisions about what to consider the mainline freeway, and which ramps to reconfigure.
Looking at the freeway-freeway ramp layout, we can see that the northwest and southeast quadrant ramps are missing (north to/from west, south to/from east). Again this was fairly typical for that era of freeway design, but leaving out ramps is frowned up these days as it is confusing for motors and shunts high speed traffic onto local streets. The southeast quadrant ramps are more consequential, because they would connect major nodes (Hollywood to Burbank & Glendale), while the missing northwest quadrant ramps would connect lower density areas. However, it’s obvious that there’s very little right-of-way available for the missing ramps, and it might be hard for the southeast quadrant ramps to compete with Barham and Forest Lawn, which make a relatively uncongested shortcut serving these movements.
Lastly, looking at the local street ramps, things are actually in pretty good shape. The 170 has a half-diamond to the north, and the 101 has a half diamond to the west. The 134 has a half diamond to the east, though there’s a little friction between the Vineland on-ramp and Cahuenga off-ramp going east. One notable gap is that there’s no on-ramp to the Ventura Freeway west between Pass Ave and Tujunga Ave or Moorpark St, almost 2 miles, so that might be something to try to fix. Going south on the 101, the Vineland Ave off-ramp is uncomfortably close to the 101/170 merge, something we looked at fixing in a post that feels like it was written century ago.
Now, to look at the deficiencies of the interchange, it’s helpful to look at a stylized diagram showing the number of lanes and traffic volumes. Note that the ramps are drawn as simply as possible, ignoring loops and bridges, to make things easier to look at.
It’s readily apparent that the central deficiency of the Hollywood Split is that there are only 2 through lanes on the Ventura Freeway (the 134 west to the 101 north and the 101 south to the 134 east). The traffic volumes are more or less evenly split between the 134 and the 101: 68,000 from the 101 south to the 134 east and 62,700 remaining on the 101 south; 63,000 from the 134 west to the 101 north and 64,400 continuing from the 101 north. In order to handle their traffic volumes, these 2-lane ramps would have to flow full for 16 hours a day. Meanwhile, the ramp volumes between the 134 and the 170 are about half of what the movements are in the other directions.
For the purposes of this post, let’s do an updated stylized diagram showing a simple solution for this bottleneck, taking the easiest approach. Let’s bump up the ramps between the 101 and the 134 to three lanes each way. Going west on the 134, that means we just get rid of the lane drop and extend the third lane through the interchange, which would be a basic bridge widening project. This would leave us with 6 lanes going north on the 101 instead of 5 lanes; the sixth lane could be dropped at the next off-ramp, Laurel Canyon Blvd.
Coming the other direction, let’s pick up the southbound onramp from Laurel Canyon Blvd to the 101 south as a sixth lane. We can then split and have 3 lanes go to the 101 south and 3 lanes to the 134 east. We eliminate the 134 east offramp to Riverside Drive, which has relatively low volume, and let that traffic be picked up by the Tujunga Ave and Cahuenga off-ramps. That clears space for the bridge widening for the third lane to the 134 east. With 2 lanes merging in from the 170 south, we have 5 lanes on the 134 east. Rather than drop the right lane and add the HOV lane on the left, let’s just turn the left lane into a carpool lane and force ramp traffic from the 170 over, since it has a lower volume anyway.
Note that we also cleaned up the 101 north to the 170 north transition, making it 3 lanes instead of 4 lanes and eliminating the need for a lane drop on the ramp from the 134 west to the 170 north. In this instance I left the right-side lane drop and added the HOV lane on the left, but maybe the opposite approach would work.
In a future post, I’ll lay this out on an aerial, and take a deeper look at some of the other possibilities for improvements discussed above.