The Washington State Department of Transportation analyzed potential safety impacts of changes to HOV hours of operation. This report presents the results of this analysis in two sections:

• The first section evaluates changes in collision potential expected from allowing general-purpose traffic to use the HOV lanes during night and weekend off-peak hours. Safety improvements needed are described.
• The second section identifies specific spot locations on the freeway HOV lane system where either high historical collision rates or roadway design issues pose concerns for higher-traffic-volume use that would result if HOV-only restrictions were removed during nights and weekends. The design and operational issues associated with these locations are described along with WSDOT's identification of safety and design improvements that could be offered in these locations.

What did the analysis conclude?

Converting HOV lanes to general purpose use during off-peak hours is feasible from an operations and safety perspective if certain safety improvements are made prior to opening. These include shoulder rumble strips, improved striping, raised profile combination edge lines, additional guardrails and median barriers, and improved signage. These improvements would enhance traffic safety in HOV time periods and throughout the day, yielding additional benefits to the traveling public.

The total cost of converting the lanes would be about $2 million for rumble strips, raised striping, signage and other safety improvements.

What did the analysis find?

Negligible affect on merging collisions
Opening the HOV lanes to general-purpose traffic at night and on weekends would have a negligible affect on merging collisions so long as the direct access ramps remain restricted to HOV traffic.

Some safety improvements can be implemented to offset the risk of run-off-the-road collisions
Allowing general-purpose use of HOV lanes in off-peak hours will change traffic patterns on freeways. The most noticeable change will be an increase in traffic in the left lane during off-peak hours.

Travelers in the left lane are more likely to be involved in run-off-the-road collisions than travelers in the middle lanes. In a typical run-off-the-road collision, a single car strays off the freeway and hits a median, median barrier, or other object adjacent to the freeway. These types of collisions are of concern because most of the freeway HOV lane system has narrow shoulders and left side direct access ramps are being built as part of Sound Transit's Sound Move program.

Spot improvements are desirable regardless of HOV lane hours of operation policy
Significant portions of the freeway HOV system do not meet the desired federal design standards for general-purpose travel. Analysis has identified several spot locations where it would be desirable to make improvements when feasible. These improvements are recommended regardless of whether the HOV lanes change hours of operation. Accident risk at these locations is increasing as more travelers use HOV lanes. Allowing general-purpose traffic to use HOV lanes at night and on weekends would only marginally increase risk at these locations Several immediate improvements would help address accident risk at these locations. Longer-term improvements at many of these locations are included as part of more comprehensive highway improvement projects planned for the future.

How was collision analysis conducted and what did the analysis reveal?

Traffic volumes and historical trends in highway accidents are key inputs for estimating future accident rates. This section describes the steps in that analysis.

Will traffic distribution change if HOV lanes are opened during off-peak hours?
Currently, freeway HOV lanes operate with low vehicle volumes in the off-peak periods, especially at night. This traffic pattern is important to freeway safety given the large percentage of HOV lanes on Puget Sound freeways that have narrow shoulders. At night, for example, the nearly empty HOV lane provides an additional safety buffer to travelers in the other lanes. In addition, planned transit direct access ramps would allow transit to enter directly into the left hand HOV lanes. During off-peak hours, the merge into lightly traveled HOV lanes would be easier.

If the HOV lanes are opened to general-purpose traffic during off-peak hours, traffic currently using the general-purpose lanes will spread out. This is illustrated in the graphic below. It is important to understand that the total volume of traffic during off-peak times is not expected to change as a result of revising HOV lane hours of use. Lane volumes will remain much lower than peak period volumes. Based on study by the Washington Transportation Research Center (TRAC), traffic will shift to the left, but the left lane would still carry less volume than the other lanes.



This traffic shift creates the potential for more merging (assuming left side transit direct access ramps) and run-off-the-road accidents because more cars will be closer to the edge of the road (rather than in center lanes of the freeway). Left side traffic lanes also tend to move faster than the other lanes.

How were lane volumes estimated?
Future HOV lane traffic volumes were estimated by using traffic forecasts for 2007 and by estimating how much general-purpose traffic will shift into the HOV lanes based on TRAC research. This methodology has been applied to 160 lane miles of the Puget Sound Freeway HOV system. This comprises all parts of the HOV system that are being considered for revised hours of operation in this study.

How were historical collision trends identified?
For this analysis freeway system crash history and trends from 1980 through 2000 were evaluated using the state accident reporting system. This evaluation included a survey of traffic safety prior to the construction of the HOV system. Crash concentrations by freeway segment (per hour of the day, day of the week), both in total and by type of collision, were summarized.

As is the case in other urban areas nationwide, collision trends in the Puget Sound freeway system show a maturing system with correspondingly greater numbers of accidents in the peak periods. This is both a function of congestion as well as the greater number of vehicles traveling during peak commute times. Current collision trends indicate that as congestion grows, collision experience will also increase.



What types of collisions were analyzed?
The concern in this analysis is increased left hand lane volumes. As a result, two accident types were identified for modeling: merging collisions and run-off-the-road collisions. The Federal Highway Administration's (FHWA) Roadside Safety Model was used to predict change in run-off-the-road crashes for the late night and weekend time period. A summary evaluation analyzing the change in merging accidents in the system was also performed.

Why didn't the analysis examine opening HOV lanes to all traffic midday?
In recent analysis, detailed modeling indicated significant and growing traffic volumes in both general-purpose and HOV lanes midday. This includes transit and carpool use of HOV lanes midday. Due to increasing traffic volumes, HOV lanes most corridors provide a significant midday travel time advantage. If HOV lanes were opened to all traffic midday, this travel time advantage would be lost. In some locations carpools and buses would be stuck in congestion. Although not specifically analyzed, the number of potential merging collisions in a midday change option is predicted to be higher because of higher levels of congestion.

There is no evidence to suggest that further study would resolve problems with midday use. Therefore, midday operation has not been analyzed at the same level of detail in this report.

What did the analysis reveal about merging collisions?
Overall, merging collisions represent a fairly small number of accidents on the freeway system. In part, this is due to the implementation of ramp metering in the region. Merging and lane changing collisions have been reduced by 20 to 50 percent where ramp metering has been installed. This type of collision is a focus for this study because left lane HOV direct access projects are designed with special design guidance WSDOT developed in coordination with FHWA and Sound Transit. Potential impacts resulting from increased traffic in the HOV lanes are an issue of particular interest to all of these agencies.

Study of the region's merging accidents recognized that they are a subset of a larger problem at ramp merges that includes lane changing, merging, rear end and loss of control collisions.



A method of evaluation was developed through discussion with WSDOT and FHWA safety experts. The analysis concluded there would be a very small increase in merging crashes (in the range of one to two crashes per year) if the HOV mainline were opened to general purpose traffic at night and weekends in 2002. In 2007, as weekend congestion grows, that number could increase but would remain small. Although not specifically analyzed, the number of potential merging collisions in a midday change option is predicted to be higher because of higher levels of congestion.

What did the analysis reveal about run-off-the-road collisions?
Run-off-the-road collisions represent approximately 16 percent of all accidents on the urban interstate highway system in Washington State. Rates are similar for freeways in the Central Puget Sound area.



Typically, run-off-the-road accidents are among the most severe collisions on the highway system because of the speed of impacts and objects hit. In the HOV system, guardrails or concrete barriers help to reduce the severity of these accidents. Run-off-the-road accidents happen with much greater frequency at nights and on weekends, making them of particular interest in this evaluation.



There has been a great deal of research on run-off-the-road collisions across the country. FHWA developed a computer model that projects the number of these kinds of crashes based on traffic volume, roadway widths, types of objects hit and traffic speed. WSDOT engineers used this model to look at the changes in the number of collisions that could be expected if more cars used the left HOV lane at night and on weekends. If HOV lanes are opened to general-purpose traffic during late night and weekend hours, the model projected a six percent increase in the total number of run-off-the-road collisions that occur at any time. The night and weekend option would each account for about a three percent increase in run-off-the-road collisions.

What safety enhancements would help prevent run-off-the-road collisions?
Because higher HOV lane volumes are expected to somewhat increase the probability of run-off-the-road collisions, a number of HOV lane safety improvements would be recommended if the lanes are opened to all traffic at nights or on weekends. These improvements would not only offset the potential impacts of a change of HOV lane hours of operation but would also enhance safety when the lanes are reserved for high occupancy vehicles.

The number of run-off-the-road accidents could be reduced by providing a raised profile combination edge line and rumble strip on the left side of freeway HOV lanes. In other locations nationwide, rumble strips reduced run-off-the-road accidents by as much as 30 percent. At least a 10 percent reduction is a common result. Ground-in rumble strips adjacent to raised line lane markers would be added where feasible. In addition, enhanced signing and delineation would be added to some spot locations. To further reduce the severity of run-off-the-road collisions, spot improvements to the freeway roadside would be made. Total cost for this work is estimated at approximately $4 million.

These safety enhancements would largely offset the anticipated impact of increased traffic volumes in the HOV lanes in 2007. WSDOT is currently discussing these possible improvements with FHWA.

Are there specific locations where freeway design is a special concern?

Certain spot locations merit special consideration when deciding whether to open HOV lanes to all traffic during nights and weekends.

How were these spot locations identified?
WSDOT considered design factors and collision incident rates to determine spot locations that merit extra scrutiny.

Spot locations where HOV-related design challenges should receive special consideration were identified. Design factors considered include the number of lanes, width of the shoulders, location of fixed objects, steepness of the hills, degree of curvature, number of interchanges, how sharply on- and off-ramps are attached to the main freeway and others.

Spot locations or corridors with high accident rates were also identified. WSDOT regularly identifies high accident locations by calculating the average collision incidents by highway segment and type of highway. Based on these data, specific locations are identified that exceed the average for that type of highway (known as high accident locations) and segments where accidents are clustered (known as high accident corridors).

Two freeway HOV lane segments were identified as high accident corridors: I-5 between I-90 and SR 520, and on SR 520 between I-5 and I-405. Additionally, two locations on the HOV system mainline were identified as high accident locations: the northbound and southbound independent HOV alignments on I-5 at Southcenter.

Locations such as the shoulder HOV lane on SR 520 west of I-405, the I-5 express lanes, ramp meter bypass lanes, direct access ramps and freeway queue jumps have already been screened from further consideration for changes to operating hours because of geometric and operational issues. This spot analysis identified the following additional locations that require additional examination and design study.

I-5 independent HOV alignments in the Southcenter areas where HOV traffic is split from general-purpose traffic
Both the north and southbound independent HOV alignments at Southcenter and SR 599 have been identified as high accident locations. Revisions to the independent alignments in the vicinity of Southcenter will be incorporated in the I-405 project. In the vicinity of Southcenter a longer, parallel off-ramp connection would be built for the southbound independent alignment. Adjustment to the northbound independent ramp alignments to extend the gore area would also be developed.

I-5 Downtown Seattle southbound HOV lanes under the Convention Center and Freeway Park
The HOV lane on I-5 southbound travels through an area with traffic accessing downtown Seattle. This segment runs below Freeway Park and the Convention Center lids for a significant portion of its length.

Additional left-lane traffic in this location could increase the amount of driver lane changes and would put additional traffic closer to the left barrier. Opening this section to all traffic would not be recommended.

I-5 Southbound at Northgate HOV at express lane entrance
At this location, the HOV lane ends and becomes a general-purpose lane as it enters the express lanes southbound. When the express lanes are closed, all traffic in the HOV lane must merge right and continue southbound on I-5. During weekend and weekday afternoon hours, I-5 southbound is often congested across general purpose lanes from 145th Street to 65th Street. Additional traffic in the left lane at this location would increase the amount of traffic merging at this location, possibly increasing congestion and conflicts.

No project is currently funded or proposed for funding which will address this location. Reconfiguration of the northbound express lanes connection to the I-5 mainline and enhanced electronic signing and illuminated pavement markers southbound would be proposed to improve the performance of this merge, a general benefit to the location at all hours for all traffic configurations. Total cost for these projects is estimated at $5-10 million.

I-405 HOV lane Renton S-curves
The S-curves have been improved greatly over the facility in place prior to the construction of the HOV lanes. This area remains a difficult spot location because of the natural terrain that I-405 needs to fit within. Placing additional traffic on the left side of the roadway would move it closer to the barriers along the S-curves. The I-405 Corridor Improvement Program would eventually help improve the geometrics at this location.

I-5 between Southcenter and Downtown Seattle (specifically, northbound at the Duwamish curves and southbound between downtown Seattle and Spokane Street, next to the large retaining wall)
The alignment of I-5 between Downtown Seattle and Southcenter is tightly constrained. It fits within a fairly narrow corridor with concurrent curves and hills. Roadway shoulders on the left are narrow, close to walls and close to drainage inlets, creating more obstacles for drivers than in most other areas. Additional traffic in the left lane at night or on weekends would place more traffic during these hours closer to the barrier and cause additional sight restrictions. In addition, express lane traffic must merge quickly into the southbound HOV lane at the access location from express lanes southbound to the mainline. If HOV lanes were open to general-purpose traffic during off-peak north of the express lanes entrance, this merge would be more difficult because of higher traffic volumes in the left lane. No project is currently proposed to improve the geometrics in this corridor. The edge treatments proposed for run-off-the-road collisions would improve safety performance on this section for all users at all times.

What data sources and assumptions were used in this analysis?

This analysis of safety and operational issues is based on the following assumptions regarding HOV lane hours of operation policy and impacts on select corridor safety and design standards:

Year 2007 traffic forecasts
Daily volume forecasts were obtained from the PSRC 2010 traffic model. Volumes were adjusted by using a straight line interpolation between existing traffic volumes and the 2010 projections to estimate year 2007 traffic. Weekend and nighttime off-peak data was estimated by evaluating existing traffic distributions throughout the day.

No additional trips generated by opening HOV lanes nights and most weekend hours
It was assumed that no additional trips would result from opening the HOV lanes to general purpose traffic during off-peak hours of operation because of the lack of congestion during late night and most weekend hours. If the midday option for lifting HOV restrictions received more detailed consideration, there should be an evaluation of the potential of additional trips on the system at this time.

Data uses screenline locations
Data is based on screenline locations unless otherwise specified. "Screenline" is a term used to describe a point where vehicles and passengers are measured across the facility.

Speed and historic traffic volume data
Speed and historic traffic volume data was collected from lanes by roadway sensors maintained by the WSDOT Northwest Region Traffic. The same loops provide the real-time traffic flow data that is displayed on the WSDOT live traffic website (http://wsdot.wa.gov/traffic/).

Traffic modeling
Traffic modeling was conducted by the Washington State Transportation Research Center (TRAC) at the University of Washington.

Accident data
Accident data was obtained from the Washington State Traffic Collision Data Base providing by the WSDOT Transportation Data Office.