Talking Traffic Episode 22 – Ramp Meters
Hello and welcome to Talking Traffic episode 22. Today is Monday, July 21, 2008. My name is Bill Ruhsam and today’s episode is about Ramp Meters!
Ramp meter? What is a Ramp Meter? Is that something that they use to extract your hard-earned cash by tolling you? Or is it something that is used to weigh your car to make sure you’re under the load limit?
Bzzzt! The answer is, none of the above. Ramp Meters are one small piece of what is usually called an Intelligent Transportation System or ITS. ITS is a blanket term for anything electronic designed to reduce congestion or increase capacity on a roadway. Variable message boards, real-time traffic information, even parking-lot occupancy information can all be called Intelligent Transportation Systems.
Ramp Meters are a key tool in the traffic engineer’s ITS kit bag. They function as a valve on the flow of traffic entering a limited access highway. A signal light, just like at an intersection traffic signal, alternates between green and red, permitting one car to flow down the ramp at a time, rather than a huge clump of vehicles. Ok, I hear you ask, why do we want to slow down the traffic on the highway access ramp? Won’t that back up traffic on the ramp? Yes, I answer, you’ve come to the core of the matter, but let’s talk about congestion on the highway, first.
Limited Access roads get congested for only a few reasons. the most basic reason is because there are too many cars on the road, but there are others. Ramps, both off ramps and on ramps, cause congestion points because there is interaction between vehicles staying on the highway and those getting off or on. Off ramps have a congestion area because of vehicles slowing to take the ramp, or changing lanes to move over. ON-ramps show congestion because the entering vehicles have to accelerate to highway speed and then find a gap to move into traffic. This causes vehicles already on the highway to slow and therefore, congestion! When you have a flotilla of vehicles rampaging down the ramp with intentions of entering an already congested highway it becomes very difficult for the merging vehicles to slot in without causing drastically negative impacts to the highway. One way you can address this on-ramp rampage is by limiting how many vehicles can enter the highway at one time.
Ramp meters allow a steady, metered, flow of vehicles to proceed onto the highway, allowing each of them time to accelerate and find a gap in the traffic stream. This smoothes the flow on the highway, reduces the chances of collisions at the ramp/highway interface…and can cause serious backups on the streets that feed the highway ramps.
yes, unfortunately, Ramp meters can have bad impacts on the roads surrounding the ramps. The Ramp Meters delay the flow of vehicles onto the highway which subsequently delays the flow of vehicles onto the ramps. This is in a good cause, though! It might take you a bit longer to get onto the highway, but once you’re there, the time you’ll spend on the highway will go way down.
Ramp meters work! Despite the possible congestion on side streets. A study by the Minnesota dept. of transportation turned off the ramp meters in the Minneapolis/St. Paul area after studying the conditions with them on. The results were, and I quote:
* A 9 percent reduction in freeway volume.
* A 22 percent increase in freeway travel times.
* A 7 percent reduction in freeway speeds, which contributed to the negative effect on freeway travel times. The reliability of freeway travel time was found to decline by 91 percent without ramp meters.
* A 26 percent increase in crashes, which was averaged for seasonal variations. These crashes broke down to a 14.6 percent increase in rear-end crashes, a 200 percent increase in side-swipe crashes, a 60 percent increase in “run off the road” crashes, and an 8.6 percent increase in other types of crashes.
Those are some significant findings. I’ll say it again, Ramp Meters are good things.
The inspiration for this particular podcast is local to me. Metro Atlanta has used ramp meters in limited numbers since 1996 when the Olympics were here. However, we are about to go full steam ahead with ramp meters at what seems like every single on ramp within 20 miles of downtown. A few have been activated already and there’s some growing pains; like any other system, it needs to be tweaked for full usefulness. That’s something else I’ll say: if you have ramp meters going in, please give the engineers a bit of time to work out the bugs. Nothing works perfectly right the very first day. In the long run, the time you spend on your commute will go down, and that’s the name of the game.
It’s obviously been a bit of time since the last episode went up. Life got in the way. You can probably expect to see Talking Traffic at two week intervals for the near future, although I may go on the occasional binge and publish weekly. We’ll see. If I have lots of good topics or questions, I’ll go more frequently.
This podcast is distributed under a creative commons 3.0 attribution non commercial no derivatives license, so please distribute it to your heart’s content, but don’t sell it or change it and please remember to credit me and Talking Traffic.org. If you have any feedback or questions, you can hit me with an email sent to Bill at Talkingtraffic.org, or you can leave a comment on the show notes at talkingtraffic.org.
The music you hear is by Five Star Fall and can be found at Magnatune.com. Thanks for listening and we’ll see you next time. Have a great week.