Ramp Meters

Ramp meters control the flow of traffic entering the freeway.

Ramp meters protect our most important roadways.  There is a hierarchy in roadways as each serves a different purpose. The Interstate system, specifically, is federally regulated to meet the needs of the Country.  Freeways that are not Interstates, such as SR-201, Legacy Parkway and West Davis Corridor, are critical to meeting Utah’s needs as a State.  Utah’s economy is directly related to Utah’s transportation system meaning mobility is necessary for commerce.  So it's important to protect mobility for all of us.  But freeways have a finite capacity.  Ramp meters are used to protect the flow of traffic on the freeway as much as possible when rush hour traffic pushes that capacity to its limits. Without ramp meters, our freeways would quickly come to a complete stop every day when everybody needs to get to and come home from work. Consider the following example: if you are pouring rice from a box and you quickly tip it over all at once, the amount of rice that comes out of the opening is very little because all of the rice crams together at the opening all at once.  But if you control the amount of rice that comes out, you can empty the box much quicker.  Ramp meters work the same way.  By controlling the amount of traffic that enters the freeway, we can achieve higher flow rates.

Ramp meter timing is determined based on the need to protect maximum flow rates for the current traffic conditions.  That means the timing changes as congestion builds and dissipates.  UDOT uses radar detection to measure vehicle speeds and traffic flow on both the freeway and the ramps joining the freeway.  In order to prevent the freeway from failing as much as possible, the ramp meters slow the flow of traffic entering the freeway.  Many of our ramp meters operate as a stand alone system, but as UDOT's freeways have become more congested, UDOT has begun to group ramp meters together to operate as a team.  This means that as one ramp meter begins to develop too much queueing on its ramp, it reaches out to its neighboring ramp meters for help.  The upstream ramp meters will respond to that call for help by slowing down their flow rates too.  However, there is also a balance between protecting the freeway and grid-locking the arterials feeding into it.  So sometimes a ramp meter will develop such severe queues, despite the help they get from their neighboring ramp meters, that the ramp meter has no choice but to release its queue onto the freeway and start over.  This is why ramp meters are a tool to help, but are not a solution to all congestion problems.

Most ramp meters can operate by allowing one vehicle per green light.  But others have so much traffic trying to get onto the freeway that we have to run the ramp meter at a faster rate.  We have to balance how much traffic we allow onto the freeway versus how much traffic we allow to back up into the arterial signals.  But people can only react to a red light and then a green light so fast before it just turns into a flashing light and becomes ineffective.  So allowing two vehicles per green gives us the ability to run the ramp meter at a faster traffic flow rate. And only some ramps need this higher flow rate because each ramp experiences different levels of demand. 

Ramp construction has to accommodate physical limits like spacing between interchanges, right of way acquisitions and acceleration length requirements.  There are limitations to the number of lanes that can safely merge onto the freeway in a set amount of space.  So we aren't always able to build the ramps to the length we would otherwise prefer.  When UDOT builds or modifies interchanges, the ramps are built to handle 30 year forecasted traffic demands.  Forecasts are based on large networks of data and historical growth patterns.  A lot of data and analysis work is put into these forecasts.  But they still are a forecast, not a guarantee.  And eventually those horizon years approach and pass by and we need to plan for further into the future and adjust accordingly. 

Although freeway capacity is usually only a rush hour problem, there are other reasons for the freeway to be restricted.  Sometimes there is a crash blocking lanes or a construction zone that has closed lanes temporarily.  Nearby ramp meters may be activated to prevent the freeway from dropping below optimum capacity and developing congestion conditions.

Ramp meters are built for sections of freeway that are forecasted to experience congestion during the next 30 years.  Once that congestion becomes a consistent pattern, those ramp meters are turned on and scheduled to operate regularly.  Ramp meters protect the flow of traffic onto the freeways.  So turning them on helps prevent the freeway operations from breaking down.

Yes! Without ramp meters, the flows of traffic onto the freeway would be uncontrolled.  Freeways have a finite capacity.  After the freeway reaches that maximum capacity, traffic conditions become very volatile and can quickly drop to failure with stop-and-go conditions that will remain until the traffic demand drops.  Ramp meters help protect the freeway capacity while still attempting to balance the conditions on the arterials feeding onto them.  No system is perfect, but the ramp meters help us manage the system that we have.

Ramp meters are a tool to mitigate congestion but they are not a perfect solution.  We cannot meter traffic to the point of grid locking our arterial network so it is a balance.  There is also an interesting phenomenon in traffic engineering that is a little counterintuitive.  It would seem that maximum capacity is when traffic is operating at freeflow speed limit conditions.  But that isn’t actually true.  At freeflow conditions, there is a lot of extra space that isn’t being used.   Maximum capacity is achieved when we have the most amount of vehicles moving through a section of road in a given time.  That optimum capacity level exists in a slightly higher density than what is perceived to be freeflow and it is pretty tough to maintain too.  As additional vehicles enter the freeway, the available space between vehicles begins to drop. As vehicles move closer together, the density of the freeway increases. But often, just as we get to that maximum capacity, the density doesn’t stabilize, it continues to increase beyond the ramp meters’ ability to control it.  The higher densities restrict the ability for vehicles to maneuver around slower vehicles and into the lanes they need to navigate to.  So people slow down and the flows drop below optimum.  As the freeway slips beyond that optimum capacity, the traffic operations begin to fail.  Vehicles slow down even more and enter a slow-down and speed-up condition which will ripple backwards through traffic.  This ripple effect, or "shock wave", robs the freeway of capacity that it would otherwise have.  Have you ever been stuck behind a crash, and by the time you pass it, the scene has already been cleared but there is still congestion?  You were experiencing a “shock wave”.  Eventually, at failure, the freeway speeds will fall to zero.  The freeway cannot recover to optimum capacity until the demand drops far below the optimum capacity.  If we protect the number of vehicles on the freeway, we can maintain that optimum capacity and get more vehicles through the freeway than if we allow traffic to feed onto the freeway in uncontrolled volumes.  It's a constant battle to protect capacity and ramp meters are doing a lot of the heavy lifting but they are not a perfect solution to a very complicated system problem.