In this introductory post on urban freeway improvements, the elimination of loop ramps and slip ramps was identified as some of the lowest-hanging fruit. It’s relatively cheap to do, and it makes things much better for bikes and pedestrians by getting rid of long, skewed crosswalks and road geometry that encourages drivers to speed. Since loops take up a lot of real estate, it also frees up a decent amount of land for development. Inspired by a recent comment, here’s a look at two interchanges on the 10: La Cienega and La Brea.
The westbound ramps at La Cienega are already in a tight diamond configuration, so there’s nothing to change there. The eastbound ramps are in a cloverleaf configuration, albeit a modified one, since Venice cuts through the vicinity. Thus, the northbound La Cienega on ramp to the 10 eastbound is a right on Venice, then a right on the ramp. The loops are very tight, with curve radii down in the neighborhood of 80’.
The basic idea here would be to remake the eastbound ramps in the image of the westbound ones. The interchange would become a modified tight diamond, with a new road connecting the two ramps between La Cienega and Venice. This would reconfigure the free movements to and from the ramps into normal city intersections, making them less hazardous for pedestrians and bikes. It would also yield new signalized pedestrian crossings of La Cienega and Venice, making things a little more walkable.
This concept uses 10’ lanes and 6’ curbside bike lanes on La Cienega. Now, I know what you’re thinking – why is there no crosswalk on the north side of the intersection of the new ramps and La Cienega? Three-legged pedestrian crossings are horrible! The reasoning is that adding a crosswalk there makes things considerably harder for the traffic engineering, and this location is a rare exception to the rule that you should never omit a crosswalk. Because there’s nothing on either side of La Cienega on that side of the ramps, there’s no chance a pedestrian would have to use all three crosswalks in lieu of the missing crosswalk. Anybody walking here is going to either a destination north of the 10, in which case they can cross at David Ave and the onramp to the 10 westbound, or a destination south of the new ramps, in which case they can cross on the south side of the intersection.
This concept adds two lanes under the freeway bridge. It looks like this might just fit under the existing bridge, because the east side has a row of parking between the existing edge of pavement and the columns.
On the traffic side, the loop ramps are both serving over 10,000 vehicles per day. Those turning movements, which are currently free (unsignalized) right turns, will be replaced with left turn phases at the new traffic signal. Excluding the crosswalk on the north side of the intersection makes that left turn easier, reducing the green time needed for the eastbound movement. Again, this is only acceptable because it’s a special situation. Here are the traffic volumes at the new intersection.
Traffic volumes are from Caltrans and LADOT. This is a really rough estimate. The approach was to guess at the worst conflict group (combination of movements that can’t proceed at the same time) for each intersection, and figure out the sum of capacities needed for each movement in the group. That’s the “g/C” column (green time divided by cycle time), representing the percentage of the total intersection capacity needed for that movement. For example, the left turn from the 10 eastbound to La Cienega northbound needs 23% of the capacity at the intersection. If the total of that column is greater than 100%, or even relatively close, the intersection is close to failing.
The existing interchange at La Brea is a full cloverleaf, with loops almost as tight as 100’ radius. However, the interchange doesn’t function like a true cloverleaf, because the outer ramps have very sharp cure radii to and from La Brea, and the offramps using the outer ramps have traffic lights instead of free-flowing turns.
The plan at La Brea would be to reconfigure the interchange as a tight diamond, using the same parameters – 10’ lanes, 6’ bike lanes. There are two options, one with the ramps tight up against the freeway, and one with the ramps intersecting La Brea near where the outer ramps do today.
The advantage of the first option is that it lets you do the same crosswalk trick as at La Brea. However, unlike at La Cienega, there’s no extra room under the freeway at La Brea. We can steal the weaving lane to get four lanes under the bridge, but that leaves only a single lane for the left turns onto the 10. Unfortunately, that probably won’t work on the traffic side.
By pushing the ramps further away from the bridge, the second option lets you fit in a second left turn lane, though due to the lane’s short length, it might be a little optimistic to assume it could be used to its full capacity. Also, because the second option puts the new development between the ramp intersections, it’s no longer acceptable to omit a crosswalk. That makes the traffic design more challenging.
Traffic volumes on La Brea are daunting – the road serves nearly 70,000 vehicles per day here, more than many freeways that are two lanes per direction. The heaviest ramp volumes are to and from the east, all approaching 10,000 vehicles per day.
This design would require the crosswalks closest to the freeway to be concurrent with the left turns from the freeway offramps, which might be difficult given the traffic volumes.
Now normally this is the part of the post where I’d suggest auctioning off the real estate to the highest bidder, as long as they agree to do something with it other than surface parking. That way you don’t end up with prime real estate owned by the government sitting vacant for years because it was impossible to come to a consensus on what to do with the land.
But you know what? F!@# it. These freeway-adjacent sites are right in GH Palmer’s wheelhouse. Just dial him up and let’s get us a few hundred Italianate apartments built. We can call them The Palude and The Catrame (the Italian equivalent of La Cienega and La Brea).
While the idea of improving these interchanges for pedestrians and bikes, and freeing up space for urban development, is appealing, the worst g/C ratios approach 1 at both interchanges. More traffic study would certainly be required to see if these plans are viable.
Politically, any plan to eliminate loop ramps is going to have to win the support, or at least the grudging tolerance, of drivers. Unfortunately, these interchanges are not the best candidates for the first project, because if the first project doesn’t go well, there won’t be any more. Back to the lab again. . .
The sausage-making behind this rough traffic analysis: I assumed the ramp volumes have the same peaking as the through movements on the 10, and 1,700 veh/hr capacity for each lane at the intersections. The critical conflict group at La Cienega was assumed to be Offramp EB – La Cienega NB – La Cienega SB left turn. At Venice, Onramp EB (from the La Cienega SB left turn) – Venice EB – Venice WB left turn. No volume was available for the last movement so it was a wild guess. The critical conflict groups at La Brea were assumed to be Offramp EB – La Brea NB – La Brea SB left turn, and Offramp WB – La Brea SB – La Brea NB left turn.