In the summer of 2003, a two-year pilot program began that allows single-occupant vehicles (SOVs) to use HOV lanes on four of the five primary freeway corridors in the Seattle area during the hours of 7:00 PM to 5:00 AM, seven days a week. After two years, the percentage and number of SOVs using the HOV lane increased at the start of the revised hours (7:00 PM) at most locations evaluated, and the percentage of all traffic using the HOV lane after 7:00 PM increased. At some locations SOV violation rates increased in the HOV lane during the transition periods (6:45 PM to 7:00 PM, and shortly after 5:00 AM); the increases were generally not large, with some exceptions at locations that also have significant general purpose PM congestion. Some locations saw a drop in violation rates. Traffic performance changes after 7:00 PM were difficult to determine, although estimates suggest a small shift toward slightly higher speeds in the GP lanes after 7:00 PM. There was almost no change in HOV lane performance after 7:00 PM.
A review of collision records for the periods 2001-2002 and 2004-2005 indicated that generally there were no adverse safety effects related to revising the HOV lane hours of operation. The evaluation revealed a reduction in daily occurrence of run-off-road collisions, including during nighttime hours. Additionally, no adverse safety effects were identified during the 7:00 PM to 8:00 PM period or during the entire nighttime when the HOV lanes are open to general purpose traffic. There was evidence that safety improvements implemented before the start of the pilot project may have contributed to a lower rate of nighttime run-off-road type collisions.
A traveler survey showed that among those who were already aware of the new policy, 31 to 45 percent associate the new hours of operation with perceived improvements in maneuverability, safety, and/or speeds, although a similar percentage of people surveyed did not perceive any change. There was sizeable support for the program among both SOV and HOV travelers, and a significant number of respondents had a more favorable opinion of the HOV system as a whole because of the new hours of operation. At the same time, only slightly less than one-half of those surveyed were aware of the revised hours of operation.