Freeway Transit and Lane Efficiency

On Saturday, the Seattle Times editorial board wrote an editorial raising questions about the freeway-oriented nature of Sound Transit’s draft proposal. The editorial is rife with misleading or just plain old wrong information. But the board does raise a worthwhile point, which is that buses on freeways can be extremely useful transit that moves tons of people effectively. Let’s look at some numbers on freeway based bus transporpation.

Typically, transportation planners estimate that the average freeway lane can carry 1,900 vehicles per hour. Given an average vehicle occupancy of 1.2 persons per vehicle, that means 2,280 people can use a given lane in a given hour. (Note that this is in freeflow conditions. When congested, freeway lanes move less vehicles per hour). Thus, speaking abstractly, a freeway lane dedicated to transit would only need to carry about 2,280 people per hour for that transit lane to be a fairly efficient use of resources. All this is during peak hours when freeway space is scarce. At times when the freeway is well below capacity, the transit lane wouldn’t effect travel times in other lanes.

Fortunately, WSDOT has travel data on key freeway corridors so we can see just how many people utilize freeway based bus transit during peak hours. For example, on I-5 southbound at Northgate during the peak morning period (5am to 10am) the HOV lane carries 48% of all persons traveling. Of this 48%, at least 73.5% are transit riders meaning that the number of transit riders through the area is about 35% of total travelers during peak period. Given that there are five lanes southbound at that location, transit is carrying about 2.7 (35 / (52 / 4))  times as many people per hour as each general purpose lane.

Transit moves people along I-5 far more efficiently despite the fact that the existing HOV lanes that transit uses, though 11 minutes faster on average between Everett and Seattle, are still congested during rush hour, with about 12 to 15 minutes of delay over freeflow conditions. In short, there are enough HOV2+ car drivers to substantially congest the HOV lane buses use. Transit ridership would probably be at least somewhat higher if the lanes were converted to transit only lanes, ensuring reliable 60mph commuting for buses in the corridor and 12 to 15 minute travel time savings. Because transit moves so many more people then the general purpose means, converting HOV lanes to transit only lanes, at least during peak, would be a very effective use of limited freeway space.

Here is a more detailed cost/benefit analysis. At 35% (conservatively) of total I-5 users during morning peak, the benefit to the average transit rider would have to be about double the cost to the average non-transit rider from the conversion of the HOV lane to a transit lane. If the average transit rider is delayed by 15 minutes, then as long as the average driver was delayed by less than 7.5 minutes with the removal of the HOV lane society would unambiguously benefit from the switch. Unfortunately it is difficult to make definitive claims about how much additional delay drivers would face with the change, except to say that the 13% of non-transit HOV users would have an additional 11 minutes of delay, which means the remaining four lanes would have to have delays of less than 5.3 additional minutes for the transit lanes to be socially preferable.

It is worth noting here that the previous paragraph undersells the benefits of transit lanes on I-5. Reliable, time competitive transit tends to develop strong ridership and 12 to 15-minute improvements in travel time along with substantial improvements in reliability should convert many more riders to transit. Indeed, speed and reliability improvements are key reasons the Everett link extension would generate 35,000 to 43,000 daily riders or about 5 to 6 times as many riders as ride the current equivalent Sound Transit express bus routes.

This analysis just looked at I-5 southbound near Northgate during morning rush. But similar claims can be made about other HOV corridors in the region with only 3 of 14 HOV corridors meeting Washington State’s reliability standard of: “an average speed of at least 45 mph, 90% of the time during the peak hour of travel.”

The capital and economic justification exists today to implement fast reliable transit covering large swaths of the region by raising the standards of HOV lanes or simply converting them to transit only lanes. And if transit becomes extremely fast and reliable in the region’s freeway corridors, then the added value of building rail lines along I-5 to Everett and Tacoma, I-90 to Issaquah and the West Seattle Freeway to West Seattle pales in comparison to the Billions and Billions of dollars that rail would cost.

It is worth noting that there is some value from building light rail over having a bus rapid transit corridor, namely operational efficiencies at scale (trains are much longer than buses) and rail bias. But these benefits, particularly capacity, are not very important for the freeway rail corridors in the ST3 draft plan. Ultimately spending billions of dollars for additional rail along freeways in lieu of converting HOV2+ lanes into transit only lanes (or at least HOV 3+) represents an inefficient use of resources, incentivizes auto-usage over sustainable mobility, and ensures that excellent transit will reach these corridors 15 to 20 years later than it could, at great cost to travelers.


2 thoughts on “Freeway Transit and Lane Efficiency

  1. I’m frankly pretty surprised tat the proportion of bus-riders in the total traffic mix at peak is that high. I guess it’s a little hard for me to conceptualize, on the spot, what a bus load of passengers in cars looks like instead.

    I am very pessimistic about being able to sell HOV3+ and other (arguable equally necessary) changes to highway usage to the public. Congestion pricing, HOV3+, BRT lanes, etc would be a great use for the existing infrastructure – but given the way the I-405 tolling/HOV system went, and the way it was received by drivers, I know better than to even dream about that.


    • So one of the problems that has plagued the I-405 HOT lanes is that because a new lane was added for a substantial stretch, choke points that were once further south are now in up in Bothell. And the state has in fact admitted that traffic is worse in those particular areas. Without a new lane that problem may not be applicable to the I-5 case or other cases.

      Moreover, while the I-405 “fiasco” may sour voters on these kinds of projects in general, I don’t think it’s quite an apples to apples comparison. Notably, in the case of I-5 (or West Seattle or I-90) there would be no new lane, so there would be no pretense of improving general purpose traffic. Now raising the HOV standards to free flow levels could improve general purpose traffic if (roughly speaking) the number of people who choose to switch to transit outweighs the number of additional vehicles that move from the HOV lane to the general purpose lane due to the higher standards, but the pretense wouldn’t be there. Instead. the project would explicitly be about the gains for the 40% to 45% of the total people on the freeway using HOV lane, outweighing the losses to the other 55% to 60%.

      I also wonder in all this why the lane volume numbers, which is really the core of my argument, aren’t published more often and whether framing projects from that perspective would eventually gain more traction. I suspect overwhelming majority of people don’t realize just how many people are riding transit along I-5.


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