Synthesis of Variable Speed Limit Signs
CHAPTER 3. VARIABLE SPEED LIMIT STATE OF THE PRACTICE
REPRESENTATIVE VARIABLE SPEED LIMIT SYSTEMS IN THE UNITED STATES
Variable speed limit (VSL) systems have been widely used in many of the States for various functional purposes. Table 2 briefly summarizes each focused VSL system that is investigated thoroughly through the literature review and agency interview. Note that planned and removed systems are not included in this table, although they are discussed throughout the report.1 The "Primary Functions" column may include any of the following descriptions:
management techniques (e.g. variable message signs (VMS), dynamic shoulder lanes, ramp metering, etc.).
VARIABLE SPEED LIMIT SYSTEMS USED FOR CONGESTION-BASED ACTIVE TRAFFIC MANAGEMENT
VSL systems used for congestion-based active traffic management (ATM) are sometimes referred to as "speed harmonization systems." The purpose of speed harmonization is to dynamically and automatically reduce speed limits in or before areas of congestion, accidents, or special events to maintain flow and reduce the risk of collisions due to speed differentials. They are usually used in conjunction with other ATM strategies such as queue warning and hard shoulder running. Note that the speed limits for VSL systems used for congestion are generally updated every 30 seconds to 15 minutes. An interval of 1 to 5 minutes was found to be the most common practice.
A regulatory, hybrid VSL system was installed along I-4 in Florida in order to efficiently manage the large volumes of traffic that regularly utilize this corridor. The system is 10.5 miles long, and there is currently no plan to change the length. This system was not built to manage traffic based on weather conditions, rather the main focus of the VSL system is to improve speed harmonization. VMS are used in conjunction with VSL to display relevant information to drivers along the roadway. Loop detectors and side-fire radar are used to collect various traffic data. The VSL system then uses this data to recommend an appropriate speed limit which can be based on the current speeds, volume, capacity, roadway geometry, etc. The operator may then accept or alter the system's recommended speed limit.
The VSL system along US 27 in Florida is 3 miles long, regulatory, and automated. Much like the VSL system located on I-4 in Florida, the US 27 VSL system uses loop detectors and side-fire radar to determine appropriate speed limits; however, no VMS is used along the corridor. The US 27 system was installed to improve safety by lowering vehicular speeds surrounding a school zone, thereby, reducing collisions and associated congestion levels. Note that there is presently no plan to alter the length of the VSL system on US 27.
A VSL system was also installed on I-285 in Georgia, where sensors capture volume and speed information to calculate appropriate speed limits based on current traffic conditions. Although the 36 mile, regulatory VSL system is fully automated, manual override may occasionally be necessary to properly handle more complex situations (e.g. work zones, etc.), which is discussed in the "Variable Speed Limit Systems Used for Work Zones" section below. Note that weather conditions are not included in the VSL algorithm.
The first deployment of VSL in Oregon was for a single intersection along Oregon Route (OR) 213, west of downtown Portland. This regulatory, hybrid system is still active due to its success, and it aims to regulate traffic and reduce congestion levels at the intersection. Note that this VSL system utilizes a single, side-mounted sign.
The advisory system along OR 217 utilizes current traffic and existing weather conditions to automatically calculate and display variable speed limits, warn of queues ahead, and estimate travel times. The final displayed speed limit depends on which piece reports the most needed condition change (weather vs. congestion). OR 217 is divided into various subzones where radar and dual loops are utilized to capture real-time speed data. The displayed speed in each subzone is calculated as the lower of these two values: 1) 85th percentile speed, or 2) speed of downstream traffic +5-10 mi/h (Mitchell, 2016). In addition to the VSL system on OR 217, the Oregon Department of Transportation (DOT) has installed active, weather- and speed-based curve warning systems at both ends of the corridor. Note that the weather-related algorithm for OR 217 is discussed in the "Variable Speed Limit Systems Used for Weather" section below.
The 13-mile, automated, advisory VSL system along I-66 in Virginia was installed to manage the high volumes of traffic and related congestion issues existing along the corridor. Note that the system is also capable of regulating traffic surrounding work zones, which is discussed in the "Variable Speed Limit Systems Used for Work Zones" section below. The speed limits are determined by a smoothing speed algorithm, which establishes the current lowest speeds along the roadway and appropriately slows upstream traffic. In addition to the VSL signs, VMS are used to display vital information to drivers (e.g., "Congestion ahead," etc.). Lane availability is also displayed along I-66 to designate which lanes are open and which lanes are closed to traffic. There is no plan to extend the current VSL system along I-66 mostly because there are already significant proposed geometric changes along the roadway. The Virginia DOT does not want to invest in more traffic flow technology until those changes are known.
The VSL system on the I-95 Express Lanes was operational in December 2014. The manual, regulatory VSL system is approximately 10 miles long. The purpose of the VSL system along the I-95 Express Lanes is to control congestion. The I-95 Express Lanes also include high occupancy toll (HOT) lanes and lane management functionality (Earnest, 2015).
The statewide, 148-mile, regulatory VSL system along the New Jersey Turnpike used to be automatic but is currently manual due to the level of sensor maintenance required after repaving. The system is used to relieve congestion and also accounts for weather conditions. Note that the weather-related portion is discussed in the "Variable Speed Limit Systems Used for Weather" section below. In general, the speed limit is manually reduced to 45 mi/h in response to a downstream incident, except when poor weather conditions are a factor. VMS are posted next to the VSL to explain the reasoning behind the speed alteration.
The automated, regulatory VSL systems along I-5, I-90 (Bellevue to Seattle), and SR 520 in Washington uses the same method to alter speeds. Downstream conditions are assessed, and the speed limits are updated every minute based on the results of the traffic evaluations. The posted speed limits may vary across lanes and throughout the corridor, although currently the system only allows differences between the HOV lane and General Purpose lanes and not between individual General Purpose lanes. In addition, VMS are used in conjunction with VSL within all three systems. Currently, there is no plan to expand or decrease the length of any of the VSL systems in Washington (I-5, I-90 (Bellevue to Seattle), and SR 520). If there are expansion or contraction plans in the future, the Washington State DOT will base that decision on engineering judgment rather than public opinion.
VARIABLE SPEED LIMIT SYSTEMS USED FOR WORK ZONES
The VSL system on I-66 in Virginia is used not only to relieve congestion, but also to regulate traffic surrounding work zones. Relevant information and warnings related to work zones are displayed on VMS message boards along the corridor. In addition, overhead lane-use-control signs are used to denote lane availability (a green arrow is displayed when the lane is open to all traffic, and a red "X" is displayed when the lane is closed to all traffic), which is especially useful for traffic surrounding work zones.
The VSL system located along I-285 in Georgia also accounts for work zones. Roadway construction is typically performed at night when traffic is lighter which consequently results in faster speeds. Georgia DOT will manually adjust the VSL when needed to reduce speeds in work zone areas.
Note that a temporary VSL system was installed along the Woodrow Wilson Bridge, but the Virginia DOT removed the system once all construction tasks were complete. This system is also discussed in the "Deactivated Variable Speed Limit Systems in the United States" section below.
VARIABLE SPEED LIMIT SYSTEMS USED FOR WEATHER
The Nevada DOT selected US 395 in Reno for the State's first VSL implementation. The highway parallels I-580 and functions as an alternate route when the interstate is closed for high winds. The VSL system is approximately 5 miles long, automated, regulatory, and activates based on wind speeds. The system has experienced some hardware/software issues related to signing such as blank signs and inconsistent posted speed limits. Therefore, the Nevada DOT is considering a temporary deactivation of the system in order to improve overall functionality.
The manual VSL system located on the New Jersey Turnpike accounts for both congestion and weather conditions. The weather-related algorithm primarily focuses on visibility. Operator guidelines are provided to determine the appropriate speed limit based on the number of visible mile markers from a stationary location (e.g., 35 mi/h is used when three mile markers are visible, etc.).
The OR 217 advisory, automated VSL System not only accounts for congestion levels, but it also accounts for current weather conditions. As mentioned previously, the final displayed speed limit depends on which piece reports the most needed condition change (weather vs. congestion). The weather-related algorithm calculates appropriate speed limits based on data collected from friction factor sensors. The weather-responsive system considers many variables (e.g. visibility, grip factor, surface condition, etc.) to determine the warning message displayed to drivers.
The Tennessee DOT installed a regulatory, hybrid, weather-responsive VSL system along I-75 in Chattanooga, Tennessee. The system is approximately 9 miles long, and the Tennessee DOT does not currently plan to alter the length of the VSL corridor. Speeds are calculated based on current visibility due to fog conditions. This system reliably and instantly provides speed reduction to drivers along I-75 using environmental sensors which monitor current weather conditions. A single speed is set for the entire corridor, and a single display is used for all lanes at a particular location.
The Virginia DOT is currently designing a regulatory, weather-responsive VSL system to regulate traffic along I-77. The proposed system will be 15 miles long, and there is no current strategy to alter the length of the planned VSL system along I-77. The system will be located in the Fancy Gap Area, which is rural and has low traffic volumes. Speed limits will be determined based on available visibility levels captured by very reliable sensors. The majority of the signing will be VMS, which will post messages related to speed limits and/or traffic management.
There are two active, hybrid, and regulatory VSL systems used for weather-related issues in Washington State: I-90 (near Snoqualmie Pass) and US 2, which are 25 and 23 miles long, respectively. Currently, there is no plan to expand or decrease the length of either weather management system in Washington. A look-up table is used for both systems to manually determine the appropriate speed, which accounts for current pavement conditions, visibility, weather (i.e. rain, snow), and incidents. The VSL system also utilizes reliable sensors to calculate travel times based on speed converted from occupancy measurements. Signing is located on the roadside and/ or overhead along I-90 (near Snoqualmie Pass). In contrast, all VSL signing is located on the right side of the highway along US 2.
DEACTIVATED VARIABLE SPEED LIMIT SYSTEMS IN THE UNITED STATES
The Missouri DOT installed a regulatory VSL system on I-270 in St. Louis. Law enforcement reported that they were uncertain of current speed limits and consequently reluctant to enforce the VSL. In response, the system was changed to advisory, but driver compliance became an issue so the system was ultimately deactivated.
A hybrid VSL system was installed on the Woodrow Wilson Bridge along I-95 to regulate traffic during construction operations. However, the system was removed once construction was complete and the work zone was no longer necessary.
Minnesota DOT has temporarily turned off the VSL systems installed on I-35W and I-94 in the Minneapolis/St. Paul area. Both systems were advisory and operated automatically, but the lag in real-time data was an issue. Speed limits were determined using the average of data sent from single loops every 30 seconds. The time to do the math to get the average speed slowed down a change in speed limits based on current conditions. If the VSL systems are reactivated, Minnesota DOT will most likely decrease the length of the corridors. Due to maintenance issues with the signs, Minnesota DOT is considering either replacing them in kind or installing a single overhead sign as opposed to lane-by-lane signage. This would reduce the cost of installation as well as maintenance and operations costs.
Agency staff offered insights into important lessons their organizations learned from their experience with VSL systems (Table 3). Advice was related to overall design, algorithm, and infrastructure.
1 The VSL systems included in this synthesis are a snapshot in time as of January 2016. A more comprehensive listing of all known planned and existing VSL systems is available at the Federal Highway Administration's Active Transportation Demand Management Program website at https://ops.fhwa.dot.gov/atdm/approaches/adm_table/index.htm. [ Return to note 1. ]
United States Department of Transportation - Federal Highway Administration