I-405 HOT Lane Benefits

The Bellevue Reporter article on HOT lane on I-405 prompted the following:’’

I was interested in the information the WSDOT is spending $334 million to implement I-405 HOT lanes between Lynnwood and Bellevue with the intent to spend an additional $1.1 billion extending them to Renton.   Presumable they were doing so to generate additional funds and to alleviate congestion.

The ability to attract SOV drivers to HOT obviously depends on the benefits from paying the surcharge.   The benefits depend on the percentage of total cars legally in the HOV lanes and the number of lanes on the roadway.  For example if a two-lane roadway has one lane for HOV but only 10% of all the vehicles have more than one rider and therefore allowed to use it, the 10/90 split provides a strong incentive to pay the surcharge.  If a third of the drivers do so the difference reduces to 40/60 and dramatically improves overall traffic flow.  However, there is little to be gained for additional riders to pay extra.

For a three-lane highway the original 10/45/45 flow and 20% of the SOV drivers opt to pay the surcharge the resultant split is 28/36/36.  Its unlikely additional SOV riders will pay the surcharge and the benefits to flow are reduced.  Similarly with four lanes the initial 10/30/30/30 split, 10% paying the surcharge gives a 19/27/27/27 distribution with little congestion benefit.  

Its clear more lanes and more HOV riders reduce not only the revenue from HOT but also the surcharge effect on congestion.   It isn’t clear what the WSDOT assumed for the number of HOVs.  With 20% HOV, an SOV driver would benefit very little on a 4-lane highway.  However, with 10% or fewer HOV drivers they should at least consider simply doing away with the HOV lanes and use the $1.5 billion HOT funds elsewhere.