Daily Archives: August 11, 2013

Principles for Freeway Improvements

Historically, freeway capacity improvements have often been made without putting too much thought into trying to get the most out of existing facilities. Problem: congestion. Solution: MOAR LANES. I think this is for the same reason you see streetcar projects popping up all over the place these days – other people’s money. For a long time, the federal government paid for 90% of freeway construction costs, so states didn’t really care how they spent the money. This has led to maintenance backlogs for states that built more than they could maintain, but that’s a separate problem.

Some readers are no doubt of the opinion that in the long run we should get rid of urban freeways. Let’s leave that debate aside for the moment and note that freeway removals are long-term projects and can be expensive, while the things I’m talking about here are short-term things we can do for cheap.

So, smarter freeway projects. Let’s assume you’ve already taken things like competing against transit and the arguments against new freeway capacity into consideration.  Here’s what you need to pay attention to when you design your project.

Make Things Better for Pedestrians and Bikes

At the end of freeway ramps, cars transition between an exclusive, high-speed facility and local streets, with lower speeds and competing uses. This means ramps are therefore prime locations for terrible incidents with drivers striking pedestrians and bikes. Therefore, it’s important for off-ramps to send a strong message to drivers: you’re not on the freeway anymore, and for on-ramps to send a similar message: you’re not on the freeway yet.

The best way to do this is with design features that force drivers to slow down or stop at the ramp terminus. A hard stop forces you to reset your expectations. To that end, here’s a quick assessment of how different types of interchanges perform:

  • Cloverleaf: probably the worst, since all turns to and from the freeway are free-flowing. Drivers coming off the freeway are likely to keep going too fast, while drivers entering the freeway are likely to accelerate into the ramp, increasing the chance of right-hooking a pedestrian or bicyclist. In addition, the ramp geometry results in long skewed crosswalks or the need to try to force pedestrians out of their way to a shorter crossing. If you try to cross a ramp like this with any regularity, you know how scary they can be.


Cloverleaf: the 710 at Willow. Note long & skewed crosswalks, large ROW needs, and weaving movements on freeway.

  • Partial cloverleaf: a little better, since it probably introduces a traffic light, but still has some free rights. A four-ramp partial cloverleaf is better than a six-ramp.


Six-ramp partial cloverleaf: the 91 at Lakewood. Less ROW and no weaving on the freeway, but still some free rights. In the SW quadrant, we have a crosswalk that forces pedestrians to take a circuitous path. In the SE quadrant, we have a skewed crosswalk.


Four-ramp partial cloverleaf: the 405 at Hawthorne. Note that the interchange ROW is tight to the freeway. The ramps intersect Hawthorne at a tight right angle, and this location has no right on red. This forces drivers to stop and reset.

  • Single-point urban interchange (SPUI) and diverging diamond: these interchanges were designed to move as much traffic as quickly as possible. Pedestrian and bike facilities are an afterthought, and to be blunt, they suck. SPUIs and diverging diamonds might be ok in a suburban or rural context, but they don’t belong in cities.


SPUI: the 10 at Archibald. The ROW is pretty tight, but the pedestrian crossings are skewed, and drivers are encouraged to speed by free rights.

  • Tight diamond: in this design, the ramps intersect the street in essentially the same configuration as a regular intersection. All traffic, including right turns, must stop before proceeding. This is the safest design for bicyclists and pedestrians, so when urban freeways are reconfigured, this is the design that should be used. Note that a properly designed partial cloverleaf can do this job just as well.


Tight diamond: the 405 at Culver. Minimal ROW impact, and the intersection of the ramps with Culver is like a normal intersection.

Reduce the ROW Impact of the Freeway

Freeways make gaps in the urban fabric. In newer areas of development that have grown up around freeways, it’s not that big of an issue, but in cities, freeways can be real barriers. Many older freeways were designed with a callous disregard for context, so any reconstruction projects should try to fix these problems.

Happily, there’s a lot of synergy between the goal of making things better for pedestrians and bicyclists and the goal of reducing the ROW impact. The interchange designs that minimize ROW impacts are also the ones that are best for pedestrians and bicyclists. If you’re in a dense urban area, there’s really no reason to ever build a loop ramp. The marginal benefit to traffic is just not worth it. The only exception to this is where the freeway crosses the intersecting road at a skew, like in the four-ramp partial cloverleaf shown above.

Make the Most of Existing Capacity

Adding lanes is the “dumb” solution, because it requires the least amount of thought. You don’t have to do any critical thinking about what’s causing the congestion. But adding lanes is also usually the costliest solution, and the most disruptive to the city, so adding lanes should be the improvement of last resort. That’s not to say it never makes sense. For example, where the 5 drops from five lanes to three lanes in La Mirada, it’s pretty clear that the inconsistent number of lanes is the source of congestion. (Note that theoretically, you could fix this by adding lanes in LA County or removing lanes in Orange County. If we want to do the latter, that’s fine, but we need to explain how we will accommodate the travel demand.)

However, in many cases, congestion is being caused by deficient design on the existing freeway. These issues should always be analyzed and addressed before adding lanes. The most common deficient design feature is inadequate weaving distance. This is a major source of congestion on many older urban freeways, which were built when the understanding of freeways was primitive, and have entrance and exit ramps placed too close together. Some examples of this in Los Angeles are the 10 between Western and the East LA interchange, the 110 between Florence and Adams, the 101 from the East LA interchange to the Hollywood Split, the 405 between Inglewood and the 110, and the horrible weaves on the 110 between the 10 and the Downtown exit, and between 3rd St and the Four-Level.

There are three options for addressing weaving problems caused by close interchange spacing:

  • Eliminate some of the on and off ramps to increase interchange spacing. This is the cheapest option, and it may be possible to recoup some costs by selling the old ROW.
  • Add auxiliary lanes or collector-distributor lanes. This is a moderate cost option aimed at easing, but not eliminating the weave.
  • Braid the ramps. This is an expensive solution requiring bridges, and probably ROW.

There are two key questions in choosing an option: how important is it to provide ramps to all these surface streets, and how much money do you want to spend? If you really need access to all the streets, auxiliary or C-D lanes will help. Braided ramps should be reserved for only the heaviest volume locations.

In general, I think removing ramps is underappreciated as a viable option. You help solve your freeway congestion problem. You save money. And you get to remove through traffic from neighborhood streets that probably shouldn’t have had ramps in the first place. If your interchanges are less than a mile apart, odds are you’re dumping freeway traffic into the wrong places anyway. Removing ramps is a way to improve neighborhoods. What’s not to like? We should be removing more ramps.

There’s also some synergy to be had with the previous two goals. Cloverleaf interchanges inherently have short weaving distances between the loops, so they can also cause congestion. They also take up a lot of ROW. They should be replaced with other interchange designs, preferably tight diamonds or, where the freeway is crossing at a skew, a tight partial cloverleaf. The 710 is littered with 1950s-era cloverleaf setups, so when that project goes forward, they should rebuilt in that manner. As a bonus, turning those interchanges into tight diamonds frees up some ROW that can be sold for commercial development or whatever.


In conclusion, freeway reconstruction projects on the older LA freeways should be looking to do the following:

  • Get rid of ramps where interchanges are too close together.
  • Get rid of loop ramps that waste urban space.
  • Square up ramp junctions and get rid of free right turns to make things safer for pedestrians and bicyclists.

We’ll see these principles in action in a new post soon.