5. Trip Reliability and Travel Time Savings

Travel speed and reliability serve as the best gauges for measuring HOV system effectiveness. For Los Angeles drivers, trip reliability is the most important factor in driving, followed by travel time. As congestion increases in the Salt Lake Valley, commuters will select routes and modes of transportation based on their reliability. This section discusses speed, reliability, congestion patterns, and travel time of corridor-wide and site-specific HOV facilities. The purpose of these measures is to describe:

• HOV lane travel speeds that can be expected for a range of trip start times throughout the day
• likelihood of an average trip in the HOV lane becoming congested (with a speed of less than 45 miles per hour (mph))
• how traffic conditions change from location to location along a HOV lane and GP lane in different traffic periods
• travel time savings realized when the HOV lane is used

5.1 Corridor-wide Operational Performance

The performance measures used to evaluate the operational characteristics of the entire HOV system along I-15 are described in this section. The operational performance is discussed independently in regard to different direction and different peak periods. The operational performance was assessed with the following measures: speed, trip reliability, and travel time savings. Each measure is discussed below.

5.1.1 Travel Speed

HOV lane performance should reflect higher average speeds than the GP lanes during peak times. Table 5.1-1 assesses the average weekday HOV and GP lane location speed along I-15 from 400 South to 10600 South. The statistical results show that the vehicle speed on the HOV lanes was always higher than the speed of GP lanes throughout the day. During the afternoon peak period, the average speed on the HOV lane was 63.6 mph, significantly greater than the 51.5 mph on the GP lanes.

On I-15, speeds less than 45 mph are considered congested. In the a.m. peak period and off peak period, speeds along the corridor are above 45 mph. During the p.m. peak period, thirty-one percent of the I-15 corridor operates at or below 45 mph in the GP lanes. Only 10 percent of the HOV lane operates at or below 45 mph in the p.m. peak. Table 5.1-1 displays the speed data collected on the multiple travel time runs.

Table 5.1-1 Average Weekday HOV and GP Lane Location Speed

5.1.2 Trip Reliability

Trip reliability measures the expected range in travel time and provides a quantitative measure of its predictability. Reliable travel time allows travelers to accurately predict travel times and to budget less time for their trips. Travel time saving is another measure of corridor-wide HOV performance. It can track changes in facility performance over time and between GP and HOV lane performance. Travel times are estimated for a range of start times for trips that traverse the length of the particular GP and HOV lanes from 400 South to 10600 South. Table 5.1-2 quantifies changes in travel time on average weekdays. For all runs during the congested p.m. peak period, travel-time difference on the HOV lane was 3.9 minutes less than on the GP lane. During the off-peak period and a.m. peak period with low congestion level, the difference of travel time on both the HOV and GP lane was small. It should be noted that travel time runs occurred on days where there were no incidents on I-15. Qualitative observation show that the HOV benefit increases dramatically when an incident causes above-normal congestion on the GP lanes.

Table 5.1-2 Average Weekday HOV and GP Lane Travel Time Comparison

Figure 5.1-1 illustrates the variation of travel speeds along I-15 on the HOV and GP lane during the a.m., p.m., and off-peak periods. The figure shows that little advantage is available from HOV usage in the a.m. and off-peak times, but that HOV lane users travel at more stable and predictable speeds during the p.m. peak hours than GP lane travelers.

On average, HOV lane users experience a travel time advantage of nearly seven minutes during the PM peak period over the adjacent GP lane travelers. During the morning peak period with low levels of congestion, the HOV lane does show a users benefit of 13.4 percent in travel-time savings. In contrast, during the off peak period, the travel times along the HOV and GP lanes are almost the same.

HOV lane travel-time savings result from low levels of traffic congestion on the HOV lane during AM and off-peak commute. From the speed analysis, the vehicles traveling on the HOV lane always maintain a high and stable speed. In contrast, the GP lane vehicle speeds vary according to congestion, whether due to recurring traffic demand or non-recurring incidents.

Often HOV lanes do not operate at expected speeds relative to volume. For example, the speeds of a HOV lane adjacent to a congested GP lane are often less than the speed limit even though the flow is well below capacity. This is often a sympathy speed. To a HOV driver, the disparity in speed between their vehicle and the adjacent GP congested lane speed is uncomfortable and therefore the HOV lane traveler slows down. This can be thought of as a continuous incident. Typically a disabled vehicle on the shoulder causes the speed of the adjacent lane to slow at the point of the disabled vehicle. The congested GP lane acts as a continuous line of "disabled" vehicles that slows the HOV lane travel speeds. The greater the separation between the HOV lanes and GP lanes, the lower the impact of sympathy speeds. In Southern California, a 4-foot striped median is incorporated to provide more positive separation between the HOV and GP lane. On some freeways, a physical separator, such as jersey barriers or pylons, limits entry and exit points to the HOV lanes but further reduces the impact of sympathy speeds.

5.2 Site-Specific Operational Performance

Examining the operation of HOV lanes at specific locations shows more details about HOV lane performance. Figure 5.2-1 illustrates the average travel speed in 15-minute intervals on each lane throughout one 24-hour weekday at a sample location.