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How hot are ‘Hot Lanes’? Two ASU engineers do the math

June 21, 2007

HOT lanes — high-occupancy toll lanes — are becoming a hot topic in the search for solutions to increasing traffic congestion.

In Arizona, state legislators have considered converting HOV (high-occupancy vehicle) lanes into HOT lanes.

The switch to HOT lanes would allow individuals, during peak traffic hours, to pay to use the lanes that now are available only to carpoolers and drivers of alternative-fuel vehicles. Carpoolers and drivers of alternative-fuel vehicles would continue to use the lanes without paying a toll.

A major argument raised against the conversion is that the HOT lanes would diminish the incentive to carpool, which was a reason for building the HOV lanes in the first place. Another criticism contends that HOT lanes may as well be called “Lexus lanes,” because they will benefit only those able to afford to use them regularly.

Debate about the effects of implementing HOT lanes revolves around the success or failure of such systems in other states. What’s missing, says Arizona State University professor Ram Pendyala, is an in-depth analysis of how the proposition will effect Arizona, and the Phoenix area in particular.

“State legislators are debating whether or not implementing HOT lanes is a good idea, but there hasn’t been adequate research done in Arizona to predict how [Phoenix-area] drivers will respond,” he says.

Pendyala is a professor of transportation systems in the Department of Civil and Environmental Engineering. Earlier this year, he enlisted the help of postdoctoral research associate Xin Ye in gathering data to take an objective look at the immediate consequences of converting HOV lanes here into HOT lanes.

The two considered a 25-mile stretch of highway that carries 100,000 vehicles per day on its three general-purpose lanes and one HOV lane. They supposed that 12,500 vehicles use the HOV lane daily, with the remaining 87,500 vehicles distributed across the general-purpose lanes (about 29,167 vehicles per lane).

They used data from the 2001 National Household Travel Survey (NHTS) to determine the demand for their hypothetical 25-mile freeway at different times of the day. Data from the 2000 Highway Capacity Manual (published by the Transportation Research Board) aided them in calculating travel times under various degrees of congestion.

Based on data from the two sources, the worst congestion in the general-purpose lanes occurs between 5 and 6 p.m. when drivers in their hypothetical all-purpose lanes travel at an average speed of about 25 miles-per-hour (mph) and reach the end of the 25-mile freeway after about 60 minutes.

Drivers in the HOV lane, however, travel at an average speed of 70 mph and reach their destination point in about 21 minutes. Clearly, these drivers have a significant time advantage.

To find out how commute times and driving speeds would be affected if the HOV lane was converted into a HOT lane, Pendyala and Ye had to determine how many cars would pay to use the HOT lane.

They decided that drivers would be willing to pay 25 cents for every minute shaved off their drive time. So, if there were a $1 toll, individuals drivers would pay to use the HOT lane only if it decreased their commute time by four or more minutes. Given the capacity of a single freeway lane (about 2,200 vehicles-per-hour), an additional 1,200 vehicles could switch over to the HOT lane and save a minimum of four minutes. Any more vehicles and the time-savings would be lower.

The new travel times would be 30 minutes in the HOT lane and 34 minutes in the general-purpose lanes. So, it seems that the real beneficiaries of the HOV/HOT lane conversion are the drivers in the general-purpose lanes, whose total commute time has been decreased by about 25 minutes.

Pendyala and Ye took their analysis further.

The implementation of the HOT lane in this scenario ultimately saves 6,800 cumulative hours spent driving daily. If every vehicle hour is worth $15, the time savings can be viewed as being worth $25.5 million annually.

And this benefit is unlikely to come at a high cost for the air quality in the Phoenix area.

According to 2001 NHTS data, only about five percent of carpool trips are motivated by the incentive to use an HOV lane, most carpool or high-occupancy vehicle trips would have occurred anyway, regardless of the presence of a HOV lane.

So, are HOT lanes the cure-all for the congestion problems in the Phoenix metropolitan area?

Pendyala and Ye say no, citing the ongoing population growth and limited potential for infrastructure expansion.

But their analysis suggests that during the hours of the worst congestion, HOT lanes offer a reliable alternative for people willing to pay the toll.

The consequence for those not willing to pay isn’t bad either.

Writer: Deanna Evans

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