Where are variable speed limits in use?
Variable speed limits are used in Europe; Tennessee, Washington, Delaware, and New Jersey have some form of variable speed limits.
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How does MoDOT determine when to lower or raise the speed limit?
MoDOT bases decisions on increasing or decreasing the speed limit by viewing the occupancy of the lanes – or the percentage of how long a vehicle uses a section of highway compared to how long it should be there. If occupancy rates are increasing, that is a good determination that cars are bunching up. The engineer also takes into consideration the volume of the highway (how many vehicles are currently on that stretch of highway). Finally, they consider how fast those vehicles are going. MoDOT will consider changing the speed limits when the occupancy, volume and speed increase or decrease significantly.
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Who determines the speed limits?
The automatic computer system will make a recommendation to on-duty operations team in the Transportation Management Center if traffic occupancy rates are slowing to such an extent that would require a speed limit change. The operations staff will review the recommendation and will consider implementing any suggested change. A traffic expert will make any final decision to raise or lower speed limits, based on traffic conditions, before implementing new limits.
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How fast will MoDOT make changes?
MoDOT will evaluate and make changes to the speed limit as needed. However, traffic specialists will not change the speed limit by less than 5 miles per hour, and will not lower the speed limit by more than 15 miles per hour between any two signs on the interstate. Additionally, the traffic specialists will wait at least 5 minutes after adjusting the speed limit before making further adjustments. However, drivers should not expect the speed limit to change every 5 minutes.
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How far back will MoDOT consider changing the speed limit?
That will depend on the current situation. If it is normal congestion, there will be standard zones of reduced speeds to regulate traffic through the congested area. If there is an incident, such as a crash, MoDOT traffic specialists will consider the impact of the incident, and will adjust speeds back along the interstate to keep a steady flow of vehicles going around the incident, while minimizing the impact to people further away.
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How much will this cost?
To purchase, install and maintain the signs for two years, MoDOT will pay approximately $900,000. MoDOT will evaluate the program after two years and determine any additional maintenance requirements at that time.
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What outcome do we expect?
MoDOT intends to use variable speed limits to improve traffic flow on I-270 during morning and evening rush periods and during times of increased congestion from incidents. Initially, MoDOT will implement the variable speed limits for an initial two year period and then will evaluate the project to see how effective a tool it is for managing traffic, and whether to incorporate variable speed limits on other interstate routes in the region.
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Why is the minimum speed 40?
Even though traffic in a congested area can travel at less than 40 miles per hour, there are two reasons to keep the minimum speed limit at 40. First of all, you can get the most cars through an area of congestion if all the vehicles are traveling between 45 and 50 miles per hour. Secondly, state law has established a minimum speed limit of 40 mph on the interstate during normal driving conditions. This is to prevent slower-vehicles (such as bicycles, tractors or similar vehicles) from traveling on the system.
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Why can't MoDOT raise the speed limit above 60 when I-270 isn't congested?
Primarily, it's because of safety. Currently, when the speed limit is 60 mph, many vehicles on the interstate travel at least 70 mph. Raising the speed limit above 60 would lead to a corresponding increase of nearly 10 miles an hour above the current limit. There is a huge difference in the fatality rates between a crash at 70 mph and at 80 mph, with a corresponding decrease in the amount of time a motorist has to respond to a crisis situation.
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