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WSDOT adds electronic signs to manage lanes on regional interstate highways
Islanders of a certain age will remember the overhead lane signals on the old I-90 bridge and the approaches to it. The signals allowed traffic planners to allot an extra lane during peak commute times. The signals disappeared long ago, but with new technology and increasing traffic congestion, they are here again.
This summer, the Washington State Department of Transportation (WSDOT) will begin installing overhead electronic signs on northbound Interstate 5, State Route 520 and Interstate 90 between Seattle and Bellevue. The signs are a key component of the WSDOT Smarter Highways program, which uses real-time traffic information to improve safety and reduce congestion.
No commuter will disagree with the fact that our highways are congested. Many factors contribute to increased travel times. In fact, more factors contribute to congestion than an average commuter might realize.
Most know that daily increases in traffic volume result in congestion, as do accidents, last-second merging, and differences in vehicle speeds.
Traditionally in the United States, the solution to congestion has been to increase the number of lanes. Realistically, however, this is an expensive solution that usually does not permanently solve the problem.
Each continuous lane has a maximum capacity of about 2,200 vehicles per hour. However, adding lanes encourages more drivers to use the roadway, hindering congestion relief. Relying purely on additional lanes for congestion reduction will lead to only temporary relief.
In fact, more lanes lead to lower capacity per lane. The relationship between the number of lanes and the capacity per lane is not linear. With each additional lane, the capacity per lane decreases due to more complex traffic patterns. This includes a higher rate of lane changing and an increased probability of accidents.
Relying solely on additional lanes for congestion relief is not only expensive, but in many ways counterintuitive. There is an inverse relationship between the number of lanes and efficiency.
In the past few years, the Washington State Department of Transportation (WSDOT) has looked at factors that will help reduce congestion. WSDOT found that if drivers are aware of lane-blocking accidents downstream, they can reduce congestion by gradually merging into adjacent lanes and avoiding last-second forced merging and accidents, and when drivers know travel times in advance, they can select the fastest route and avoid contributing to already congested routes.
WSDOT research also found that if drivers traveled at a speed appropriate to traffic volume, roadway capacity can be maximized. Higher speeds decrease lane capacity, because the distance between vehicles increases.
But lower speeds also decrease lane capacity, because traffic takes longer to pass from point to point. The speed that optimizes capacity by allowing for maximum lane throughput is 45 miles an hour.
The solution to congestion is to improve awareness, manage capacity and control traffic flow.
To find a cost-effective solution that would significantly reduce congestion and improve safety, WSDOT engineers traveled to Greece, Germany, Denmark, the Netherlands and the United Kingdom in 2006 to study their traffic management systems. What they saw was “a comprehensive, extensive and customer-oriented approach to operating the system known as Active Traffic Management.”
Four years later, WSDOT is almost ready to put the Active Traffic Management (ATM) system to use on Washington highways, one of the first states to do so in the nation.
ATM is an internationally proven system that has had dramatic impacts on highway travel. Pioneering stages of ATM have been used in Germany since the 1960s. Its application throughout the rest of Europe has led to:
• Increase in throughput: 7 percent
• Increase in capacity: 30 percent
• Decrease in primary accidents: 30 percent
• Decrease in secondary accidents: 50 percent
• Travel time reduction: 20 percent
ATM is perhaps the most revolutionary traffic technology to have arrived on American soil.
This August, WSDOT will begin implementing the ATM system on Seattle-area highways, the first stage of its Smarter Highways program. The first will be I-5, in July 2010, followed by SR-520 in August and I-90 in March 2011.
ATM is designed to micromanage traffic flow on highways to increase traveler awareness, maximize capacity and streamline traffic, leading to a significant increase in traveler safety and a reduction in travel times. It will use a combination of electronic signs, sensors and cameras to manage traffic. Electronic signs over the roadway every half mile will display variable speed limits, lane statuses and real-time traffic information.
Variable Message Signs (VMS) will alert drivers of upcoming traffic incidents, such as an accident, work zone or lane closure. They will also provide drivers with travel-time information, allowing them to use alternative routes. This is known as “dynamic rerouting.”
VMS, which are also variable lane control signs, will display mandatory speed limits above each lane that will maximize the throughput of the roadway during peak time. This is known as “speed harmonization.”
These signs will also display lower speed limits prior to a backup to slow traffic ahead of time, reducing rear-end collisions. This technique is known as “queue warning.”
Variable lane control signs will tell drivers to merge into other lanes prior to an accident scene, reducing last-second forced merging. They are also able to close lanes in the event of a traffic incident, open hard shoulders as an additional lane in emergency situations, and control junctions to facilitate merging for traffic entering from on-ramps. This is called “line control” and “junction control.”
The result of the ATM system is reduced travel times, decreased congestion and increased safety. The benefits of such a system also come at a fraction of the cost of constructing a new lane. The total projected investment for the ATM system on eight miles of eastbound and westbound I-90 and SR-520 is $42 million.
On the other hand, the budgeted funding for adding two lanes on a two-mile stretch of I-405 between I-5 and SR-167 was $91.5 million.
Jason Lu is a senior at Skyline High School. He has has studied transportation systems in different cities and countries and their impact on daily commuting cultures. He is particularly interested in how real-time traffic information can improve highway capacity without expensive road building.