An unorthodox (and maybe crazy), but Really Simple idea for the CRC (and beyond)

It's time to take a little time off from the Independence Day (in the US) festivities to make a suggestion--that will likely be dismissed as crazy.  It's a proposal for the Columbia River Crossing that will strike many as absurd on its face--especially those who believe that a key goal of the project is to keep the freeway moving at "freeway speeds" for as many hours of the day as humanly possible.  It's an idea that some might not only consider foolish--but unpatriotic.  (The fact that certain ideas for bettering the country are considered treacherous in some quarters is itself idiotic in the extreme, but that's another topic...)

It's an idea that I can't take credit for... but I'll make the suggestion anyway.

A few weeks back, I had an email conversation with Jarrett Walker of humantransit.org fame on the Columbia River Crossing, a conversation which led to this post on the controversial project.  Most of the content of the email exchange went into the posting (after much editing and expansion), but one exchange was left out.  That exchange is the subject of this post.

Why is it so expensive, anyway?

One reason the project cost is so high is not the cost of the new bridge itself, but the cost to essentially rebuild I-5 and all its interchanges from SR500 on the Washington side, down to the Interstate Avenue interchange on the Oregon side.  That's a distance of almost five miles.  Why are ODOT and WSDOT so eager to pour $4 billion worth of concrete?

I-5, as currently designed, is functionally obsolete--there are numerous ramps with short distances between them, and ramps with very short merging lanes--particularly the two (from Hayden Island and SR14) immediately preceding the current bridge in both directions.  There's about seven onramps and offramps in each direction over the stretch, including an infamous 400 degree ramp (!!!) from OR99E northbound onto I-5 north.  (You cross over I5,  turn right, pass back under the freeway, and then do a 270 back onto I-5--at which point you have to merge quickly if you're not headed to Jantzen Beach).

Through the magic of such things as collector/distributor lanes and braided ramps--and a vastly widened bridge, the two state DOTs (or should that be DsOT?) intend to make I-5 once again compliant with Modern Design Standards.  Of course, it goes without saying that this sort of freeway construction requires a big footprint, a lot of concrete--meaning it does a lot of damage to surrounding urban fabric and costs a lot of money.

Whose design standards?

On my recent trip overseas, I noticed that many freeways in Hong Kong and China--both places are building them crazy, aren't built to anything remotely resembling US design standards--yet appear to be reasonably safe and functional.  A six lane footprint (3 per direction) was common, ramps were frequently closer than the 2 miles (3.2km) that DOTs and traffic engineers here prefer for ramp spacing on arterial freeways.  So I asked Jarrett, who's been in far more places than I have, about this:

Which gets me thinking:  Are US standards for freeway design to strict?  Is safety over-emphasized--a common issue with wealthy countries?  (It's interesting to compare China and India--for example, see this article).  Or is this an example of mobility values (in the highway sense of the term) trumping access?  For a highway which penetrates the downtown core of a major city, it seems to me that access concerns ought to predominate--which means more ramps and lower speeds, not designing the thing so that through traffic can blast through at 55MPH (90 km/h) during rush hour, but with a tremendous footprint that wholly disrupts neighboring communities.  Should highway engineers distinguish between "through" freeways" and "access" freeways--right now, pretty much all freeways are classified as throughput-focused arterials. 

And Jarrett responded:

Re road standards for merges, etc, I think you're on the right track.  Most standards for these things presume a "design speed," and you can reduce the amount of facility that the standards require by reducing the design speed.  For example, there is some speed (of the entire freeway) at which the southbound ramp from SR 14 would be workable.  What is it?  30 mi/hr?  It seems to me that one no-project solution -- as long as the thing isn't really in danger of collapse -- is just to lower the speed limit to whatever the road geometry can serve.  Throughout the CRC debate, any reference to highway design standards should be met with queries about design speed, and whether that speed is appropriate.

While I'm not one that favors a "no-project" solution--no project means no transitway (whether bus or rail), no decent pedestrian or bike facility, and continued interference with the Columbia navigation channel.  I'm in the "do something" camp, just not the "build a 12-lane behemoth" camp.


One foot on the brake, and one on the gas.... 


So here's a thought experiment:  What if I-5 were signed for a lower speed for a couple of miles; and the DOTs could just do a bridge replacement without having to rebuild the entire freeway as currently proposed?  (Or could build less?) What affects would that have?  Would the existing freeway, sans the current bridge, function adequately at the lower speed?

Some of the consequences would be:

• Through trips on the freeway through the 35MPH section would take about 50% longer, assuming in either case, motorists routinely travelled 5MPH over the posted speed.

• Highway throughput (per lane) might decrease slightly.  A good approximate value for the throughput of a highway is 1800 vehicles/lane-hour, which is easily derived when one considers the two-second rule.  If one vehicle passes by a point every two seconds, that's 3600/2, or 1800 per hour.  A slightly more accurate formula (accomodating for the fact that cars take up room) is 1800*x/(x+5), where x is the speed in MPH--if x is 60, that comes to 1661 vphpl, at x=40 we get 1600 vphpl.  More detailed analysis of this requires traffic engineering chops I don't possess (I'm not that kind of engineer).

• Lower speeds may turn marginal ramps configurations into acceptable ones.  The length of ramps (and of merging lanes) is dictated by the need to provide room for vehicles to safely accelerate to (or decelerate from) freeway speed, and in the case of onramps, successfully merge with freeway traffic.  For loop ramps such as ramp from SR14 to I-5 south, and any ramp with with metering signals (essentially, all of them in the project area), the portions of the ramp which precede the signal and the straightaway don't count.
• Lowering the speed limit wouldn't help the fuel economy of combustion-powered passenger cars much, and for some models may hurt slightly.  Two important factors for fuel economy of a combustion-powered vehicle are its aerodynamic properties, and its transmission.  Most passenger cars are sufficiently aerodynamic that drag doesn't dominate fuel economy until speeds exceed 60MPH (97km/h) or so--and given that, transmission gear ratios are chosen so that the car performs well in the highest gear at that speed.  A highway speed which prevents the use of high gear would negatively impact the fuel economy of combustion-powered vehicles.
• On the other hand, diesel-electric and electric powered vehicles, as well as some types of hybrids, have different considerations  Internal combustion engines generate low torque at low RPMs (and stall outright if engine speed drops too low), and thus need mechanical transmissions or torque converters to keep the engine turning at adequate speed regardless of the vehicle's speed.  Electric motors can generate high torque at low RPMs (includng zero)--thus low-speed operation can occur at correspondingly low engine speeds, with the result that fuel economy doesn't suffer from the need to downshift or idle.
• For large vehicles, regardless of the powertrain, drag will dominate fuel economy at far lower speeds.  Fuel economy for pickups and SUVs tends to peak at 40-45MPH (65-72 km/h); and larger vans, trucks, and busses peak at even slower speeds.  Any reduction in highway speeds will improve fuel economy for these vehicles.
• Speaking of such vehicles, they often take a long time to accelerate to typical freeway speeds, causing potentially more disruption when merging in and out of traffic at higher posted speeds.
• At lower speeds, the storage capacity of a highway increases (the number of vehicles which can be accommodated on a given chunk of concrete).  Storage capacity is maximized at low speeds, which is one reason traffic jams are so popular.

Where else?


Lowering the speed limit might help with some other stretches of substandard highway in the Portland area as well.  OR 217, linking US26 in Beaverton to I-5 in the Tigard/Lake Oswego area, is a notorious parking lot, owing to having about 10 interchanges along its 7-mile (11km) length. In Beaverton, there's a stretch of four consecutive interchanges (Denney, Allen, OR 8/10, and Walker) where one ramp's onramp and the following offramp are each about 1000' (300m) apart.   Last year, ODOT considered closing numerous ramps along the freeway during rush hour, to improve the highway's performance--an idea which was unsurprisingly unpopular with Beaverton and Tigard residents.

The problem with ODOT's proposal is that it focuses too much on mobility, and not enough on local access. While 217 serves an important role connecting the high tech Beaverton/Wilsonville corridor with I-5, it also serves much local traffic as well.  The area doesn't have anything resembling a fully-connected street grid, and the parallel local transit service (WES, and the 76 bus which is well-used, but infrequent and slow) leaves much to be desired. 

Plans to widen the freeway are in the works; although it is rather early in the process--and simply widening the highway will not fix the problems, as the study report indicates.  The report in question also calls for "interchange improvements"--braided ramps and such--if and when funding becomes available.

If instead of doing this--could simply slowing down the highway, to make the existing ramp configurations safe(r)--improve traffic outcomes at a fraction of the cost?  End-to-end commute times between I-5 would increase by several minutes in the worst case; but in times of congestion, the effective speed limit on the highway drops dramatically due to the numerous merge/weave conflicts--and that's without an accident occurring.


Precedents?

It is widely assumed, in the United States, that 55MPH (90km/h) or faster is the default minimum speed for freeways, without good reason otherwise.  Mitigating dangerous road conditions due to geography is often considered acceptable; Interstate 5 is signed at 50MPH (80km/h) through the Terwilliger Curves (a notorious set of S-curves on a fairly steep grade, where the freeway passes through the Tualatin Mountains just south of downtown).  However, the idea of mitigating dangerous road conditions due to excessive access seems to be out of scope--many take it as an article of faith that the proper response in these conditions is to redesign or eliminate the access in question--even if it costs billions of dollars and greatly increases the highway's cross-section.

But there are plenty of examples of freeways with lower speed limits, many of them labeled as "parkways".  In Washington DC, for instance, one finds the George Washington Memorial Parkway and the Clara Barton Parkway--both controlled access, divided highways which look and act like freeways; yet are signed with speed limits ranging from 25-50 MPH (40-80 km/h).  Even the Baltimore/Washington Parkway, a major freeway between DC and Baltimore, is signed at 45MPH for quite a bit of its length.  Yet all three roads are widely used (and widely accepted) by Washington-area commuters.


Bottom line


The bottom line for this post is the following:  Freeways perform fine at lower speeds.  The most important attribute of a freeway is not a high speed limit, but controlled access and grade separation--the free flow of traffic.  These attributes make safe travel at high speed possible--highways in urban areas which have high limits but at-grade intersections or unlimited property access tend to acquire reputations as deathtraps--or as ODOT likes to call them, "safety corridors".   However, high speeds are not necessary for a freeway to function as a freeway--and eliminating things like stoplights and driveways will go a lot longer to improving the speed of a highway than simply raising the limit.

And in the urban context, where the regional mobility function of a highway frequently comes into conflict with the access functions, attempts to simultaneously improve mobility while keeping speeds high, generally are expensive--and interfere greatly with the surrounding community.

Trading speed (or other performance measures) of for reliability is a common solution in many engineering domains, not just traffic engineering. Yet the tradeoff seems to be something which is regarded as unthinkable by many.

Which is unfortunate. We could vastly slow down our need to engage in expensive and destructive highway re-designs... if we could simply slow down.

[edited for typos]