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Seattle Traffic Questions


How do I use this site?

» How do I select a camera?
» How can I tell which way the camera is currently pointing?

What computer equipment and software do I need to view this site?

» What computer capabilities do I need to view the traffic camera pictures?

Accidents and Police

» I was in an accident near one of your cameras. Do you have a videotape of the accident?
» Is WSDOT's video used for police surveillance or investigations?
» Are the cameras able to read vehicle license plates and monitor traffic speeds?

How does WSDOT present all this cool info?

» How does the traffic data get back to the Traffic Management Center?
» How do the camera images get from the field to my monitor?
» How are the video clips made?
» Why did WSDOT choose video clips instead of streaming video?
» Where does the information about traffic conditions come from?
» How does a traffic counter or loop work?
» What kind of computer is used at the WSDOT Traffic Management Center?
» How does the traffic data get converted to a color-coded traffic map?
» How often are the color-coded traffic maps updated on the Internet?
» How are "Stop and Go", "Heavy", "Moderate", and "Wide Open" defined?
» In the map legend, what is the difference between "No Data" and "No Equipment?"
» How do the internet Traffic Pages work?
» How often are the camera traffic pictures updated?

Third party data usage

» How does WebFlow work?

Travel Time Questions

» How accurate are WSDOT's calculated travel times?
» How does WSDOT calculate travel times?
» How did WSDOT start calculating travel times?
» How are travel times being improved?

What's next?

» Why are video clips only available for some of the cameras?
» How many cameras does WSDOT have, how far is the limit of coverage, and when and where will you add additional cameras?

General questions

» What else does WSDOT do at the Traffic Management Center?
» Besides the traffic counters or loops, where else does WSDOT get information?
» What are the benefits of the video cameras?

What if my question is still not answered?

» Contact us

How do I select a camera?

Guide to the Seattle Area Traffic mapThe Puget Sound Traffic page provides access to snapshots and video clips from many of the cameras located along the major freeways in the Puget Sound area. Video clips are available at selected camera locations, while snapshots are available from each camera location. To view images from a particular camera, click on the appropriate camera icon (if using the clickable map), or the corresponding location (if using the text list). As shown in the Map Guide, cameras with both snapshot and video clips available are denoted with a round, yellow camera icon. Plain camera icons denote locations with snapshots available but no video available.

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How can I tell which way the camera is currently pointing?

This can be difficult, especially since operators are frequently moving the cameras to confirm incidents and observe ramp meters. To aid in determining the camera's orientation, we recommend using the reference images. These images are available by clicking on the "Which way is the camera pointing?" hyperlink below the "live" image. These reference images show the view from the camera when it is pointed in its most common directions.

Sometimes by looking at the relationship between the location of the camera and the roadway you can determine camera orientation. For example, if the camera is located on the West side of I-5 and the lane closest to the camera is heading towards you, the camera is pointed northbound. However, if the traffic in the lane closest is flowing away from you, then the camera is pointed southbound. The location of each camera with regard to the highway is listed underneath the picture --Right above the “Which way is the camera pointing?” link.

Another way is to look at traffic patterns and landmarks. During morning commute hours, the heaviest traffic is usually headed into Seattle, and headed out of Seattle during evening commute hours. This is not always true, however. For example, across the Evergreen Point Bridge on State Route 520, traffic is equally heavy in both directions during both commute periods. So look for landmarks such as the bridge control tower, exit signs and the topography of the lake. Exit signs and landmarks are also helpful when looking at other highways.

The I-5 Express Lanes and the I-90 Express Lanes offer another clue to camera orientation. During morning weekday commutes, the I-5 Express Lanes are open for southbound traffic and the I-90 Express Lanes are open westbound, both towards Seattle. During other hours and on weekends, they head out of the city northbound and eastbound, respectively. (Note, however, that we sometimes temporarily switch directions to accommodate traffic attending large special events.) To find out the current direction of the Express Lanes, look for the informative graphic contained within many of the traffic congestion maps. You can also get real time traffic and weather information by dialing 5-1-1 from most phones.

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What computer capabilities do I need to view the traffic camera pictures?

You need a computer and monitor that displays 256 colors (more than 256 colors will result in better quality images). You also need an Internet connection with a newer version of a world wide web browser that supports JPEG (Joint Photographic Experts Group) images. If you are reading this message and can't see the camera image, your browser does not support JPEG picture clips; you will need to obtain a newer browser.

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I was in an accident near one of your cameras. Do you have a videotape of the accident?

We are able to record video from any selected camera at our Traffic Management Center in Shoreline, but we do not regularly record our camera feeds for archiving. 

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Is WSDOT's video used for police surveillance or investigations?

We do not record video for the purpose of police surveillance or investigations. Washington State Patrol (WSP) staff also have access to our video system and can select, pan/tilt and zoom the cameras. Like the Traffic Management Center, WSP staff are able to record camera feeds.

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Are the cameras able to read vehicle license plates and monitor traffic speeds?

Our traffic cameras are not high-quality photographic images as they are not intended for photographic but rather monitoring purposes. We do not have fine enough camera resolution to read license plates on vehicles.

The cameras do not have sensors to detect speed. Instead, we monitor traffic speed, volumes and congestion with loops embedded in the freeways that use magnetic fields to sense when traffic drives over them. We do not monitor speeds of individual or specific vehicles.

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How does the traffic data get back to the Traffic Management Center?

Each field cabinet is equipped with a type Bell 202 1200 bit-per-second modem. This modem is connected to copper wires that interconnect 8 field cabinets together with a communication hub. The copper wires run up to 5 miles and the communication hubs are spaced at 10-mile intervals. Fiber optic cables connect the hubs together and to the Traffic Management Center. These fiber optic cables carry information from the traffic counters as well as the video cameras.

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How do the camera images get from the field to my monitor?

The Washington State Department of Transportation (WSDOT) currently operates an extensive closed circuit television (CCTV) system located in the greater Puget Sound area. These cameras are connected to Communication Hubs (Comm Hubs) located along the freeways. The Comm Hubs are spaced at approximately 10 mile intervals and interconnected with fiber optic cables. At the Comm Hubs, the video signals (TV pictures) from many cameras are combined using an analog technique called Frequency Division Multiplexing (FDM). Using FDM, 16 CCTV pictures are transmitted up to 35 miles over a single glass fiber.

At the Traffic Management Center, the video signals are de-multiplexed and connected to a video switch. The video switch has 448 input ports (cameras) and 112 output ports (monitors). The video switch is controlled by a HP Windows Server which is, in-turn, controlled by Operator Control Consoles. Each monitor port can be commanded to display any camera or sequence of cameras. (For the techies out there, the cameras are made by COHU, the pan/tilt units by Pelco, the video switch was manufactured by American Dynamics, Inc., and the video transmission system was manufactured by Catel, Inc.).

One monitor port on the video switch is connected to a personal computer (PC) via a Matrox Meteor video capture card. This PC acts as a traffic Video and Data Server (VDS). The VDS software was developed in-house at WSDOT. The software basically emulates an Operator Control Console and controls the video switch via the Traffic Management Center's VAX computer. The PC is a Dual 800 MHz Intel Pentium III running Microsoft Windows NT 4.0 Workstation. The application software was written using Microsoft Visual C/C++ and is a Win32 multithreaded, multitasking program. The process starts when the program commands the video switch to display a specific camera on the monitor port. When the camera is displayed, the program captures a single video frame using the Microsoft Multimedia Interface. The video frame is saved in a disk file using the Device Independent Bitmap (DIB) format. The disk file is then read back and software compresses it to a Joint Pictures Expert Group (JPEG) format. The JPEG data is written back to disk and the file's name placed in a queue for transmission to the World Wide Web (Web) server. The pictures are displayed on the Web using an HTML web page developed in-house at WSDOT. When the Web user selects a camera on the Web camera map, the HTML page retrieves the file that is stored on the Web server. Since the VDS program constantly updates the files on the Web server, the pictures appear to be real-time. The pictures are about 16-30 kilobytes each and take about 6 seconds to download using a 28.8 modem. The snapshot page will auto refresh the user's screen every 1.5 minutes using an HTML client-side pull.

Below is a picture of the path traveled by the video images, from the field to the Internet.

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How are the video clips made?

The video clips are made using the same videostreams as described above for our camera snapshots. Using a video capture card (Winnov Videum Board) installed in the same PC, we record 5 seconds of video from a particular camera. This video is compressed using the MPEG-4 encoder, and then saved as an AVI (Audio Video Interleaved) file. After each video clip is recorded and processed, the files are transferred to our WWW server where they are available for download.

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Why did WSDOT choose video clips instead of streaming video?

Video clips were chosen to minimize this feature's impact on our Internet connection and servers. We feel that 5-second video clips should be able to provide the user an idea of what traffic is like without creating an unbearable load on our Internet servers. By decreasing the resources required to provide video from each camera, we are able to provide video clips from a number of cameras instead of streaming video from just one.

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Where does the information about traffic conditions come from?

WSDOT has a network of traffic counters embedded in the roadway. These traffic sensors are six foot square or circular loops of copper wire connected to cabinets located beside the road. The traffic counters or loops are located about every half-mile on major Seattle area freeways. When a vehicle drives over a loop, it is counted, and the time that the vehicle spends over the loop is measured. The data is then transmitted every 20 seconds to the WSDOT Traffic Management Center located in Shoreline, WA.

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How does a traffic counter or loop work?

The loops are 6-foot square or circular coils of wire. A coil of wire has an electrical property called inductance. The inductance of a loop of wire decreases when a conductive material is brought near it. This is the same principle used in a metal detector. Electronics located in the roadside cabinet, measure changes in inductance of the loop. Thereby, it detects vehicles passing over the loop. The time that a vehicle spends over the loop is measured and used to determine the level of traffic congestion.

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What kind of computer is used at the WSDOT Traffic Management Center?

The main computer of the Traffic Management Center is a HP Windows Server.

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How does the traffic data get converted to a color-coded traffic map?

At the Traffic Management Center located in Shoreline, the traffic data from all of the loops is collected every 20 seconds and processed into a numerical database. Separate PC computers request portions of the numerical traffic data and plot the data on their monitors using colors to represent different ranges in the traffic data. The data is all referenced by unique identifiers. The TMS computer also stores the configuration and location of all the cabinets in the database.

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How often are the color-coded traffic maps updated on the Internet?

We attempt to write new versions of the color-coded traffic congestion maps to our web server once every minute, 24 hours-a-day, 365 days-a-year. However, during busy times, these updates can be delayed a few minutes.

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How are "Stop and Go", "Heavy", "Moderate", and "Wide Open" defined?

Their definitions are based on a measurement called "loop occupancy". This is the percentage of time that a 6-foot square loop sensor is activated, or occupied, by vehicles traveling over it. Loop occupancy is measured by sampling the loop detector at a rate of 60 times per second. Each sample results in a "loop occupied" or "loop not occupied" response. A counter is incremented once for each "loop occupied" response. After 20 seconds, the total number of "loop occupied" responses is divided by 1200 (the total number of samples in a 20-second period) and then multiplied by 100 to get a percentage. The result is known as occupancy. On the map, "Stop and Go" (or BLACK) is defined as occupancy greater than 35%, "Heavy" (or RED) is 22-35% occupancy, "Moderate" (or YELLOW) is 15-22% occupancy, "Wide Open" (or GREEN) is below 15% occupancy.

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In the map legend, what is the difference between "No Data" and "No Equipment?"

"No Data" means there is, or used to be, a data collector (field cabinet) at the site which is not currently working. The main reasons for a field cabinet not to work are: road construction (which often destroys the loops, the power connection, or the communication link between the field cabinet and the central computer), a local power outage, or, on a rare occasion, a malfunction of equipment. "No Equipment" means that data collection equipment has not yet been installed at the site. Usually, "No Data" locations will begin working sooner than "No Equipment" locations.

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How do the Internet Traffic Pages work?

A PC computer located at the Traffic Management Center in Shoreline logs in to the main computer and requests current traffic data. This PC is running a custom program similar to WebFLOW. This program plots the traffic maps in memory, and writes them as .GIF (CompuServe's Graphical Information Format) files to the State's Internet Server in Olympia. The Internet Traffic Pages link to the .GIF files and shows them to Internet users using standard HTML and HTTP: services.

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How often are the camera traffic pictures updated?

Our server downloads new camera images to the Internet approximately every 3 minutes. However, your browser controls the frequency at which the images are downloaded to your computer. Newer browsers with client-side pull capabilities will update your currently viewed image every 1.5 minutes. If your browser does not support the client-side pull function, you can manually update the image by pressing the "Reload Image" button located below the "live" image.

The date and time that each image was created is shown in the bottom right corner of the snapshot.

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How does WebFLOW work?

WebFLOW uses FTP (file transfer protocol) file transfers and a "Windows Sockets interface" called winsock. The winsock connection allows your computer to download the data files from our FTP server. Once the data files are downloaded to your PC, WebFLOW creates a color-coded graphical map of the freeway system from the data; the colors represent different levels of traffic congestion.

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How accurate are WSDOT's calculated travel times?

The accuracy of calculated travel times varies depending on congestion. The equipment used to estimate speeds on the freeway becomes inaccurate when traffic exceeds 60 mph or drops into stop-and-go congestion. Unfortunately, many of the major commutes in the Puget Sound area involve sections of freeway where speeds don't fit within this range. WSDOT verified the accuracy of the estimates by having UW students drive the route and record the time as they passed checkpoints along the way. These tests showed that the estimated travel time was accurate (within five minutes) during 84 percent of our tests.

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How does WSDOT calculate travel times?

Each route is divided into a number of sections, with a data station on each end. A speed is calculated for each section based on the data from the field and that speed is used to determine a segment travel time. The route travel time is the summation of the segment travel times. The techies out there might be interested in a more detailed explanation .

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How did WSDOT start calculating travel times?

The desire for automated travel time information most likely started about ten seconds after the first congestion map was published. However, providing accurate travel times has been a problem not easily solved, and has been the topic of many research publications. Improvements in implementation of detection equipment led WSDOT to reconsider the use of travel times as a commuter tool. Although not sufficient for many academic uses, WSDOT believes that the available data is sufficiently accurate to be useful to drivers.

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How are travel times being improved?

The most common feedback regarding the travel times is a request for more routes, and although it isn't feasible to generate a route that matches each individual commuter's trip to work, we have added
a few new routes in response to public requests. WSDOT is also improving the accuracy of the travel time estimates by designing new construction to include equipment able to more accurately determine speed and by eliminating known sources of error in our existing travel times.

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Why are video clips only available for some of the cameras?

WSDOT has been providing snapshots for several years and they have developed quite a large audience base. Because the video clips are significantly larger in size and we expect them to be a popular feature, we are starting with a limited implementation so that server and bandwidth requirements don't exceed our capacity. After we have had a chance to gauge usage, we plan to add new video clips as resources become available.

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How many cameras does WSDOT have, how far is the limit of coverage, and when and where will you add additional cameras?

Over 400 cameras now provide a bird's-eye-view of traffic along major freeways around greater Seattle. Many of these cameras also transmit images to the Internet for your viewing.

New cameras are constantly being installed and activated as we complete highway construction projects. Additional projects are in the works to further expand coverage on I-5 in the years ahead.

Map of future system expansion...

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What else does WSDOT do at the Traffic Management Center?

One of the main activities of the WSDOT Traffic Management Center is to provide traffic information to the public. The Traffic Management Center uses variable message signs located over the freeways to inform motorists of incidents, construction information, and congestion. Highway advisory radio is also used for this purpose. The Traffic Management Center operates a real time traffic and weather information line at 5-1-1 that is available 24 hours a day. To help alleviate traffic congestion, WSDOT operates the "freeway entrance lights" called ramp meters. The main purpose for the data collection is to operate an efficient ramp metering system. Things like the Internet Traffic Page, and other traveler information services are actually a secondary benefit of the ramp metering system.

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Besides the traffic counters or loops, where else does WSDOT get information?

WSDOT has a system of over 400 Closed Circuit Television Cameras (CCTV). The operators at the Traffic Management Center use the cameras to locate and verify traffic incidents and to observe and adjust the ramp meters. Information is also provided by the Washington State Patrol's (WSP) Computer Aided Dispatch (CAD) system, voice intercom links to WSP, and via radio from WSDOT Maintenance personnel.

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What are the benefits of the video cameras?

Our video cameras are a key component in our ability to manage traffic in this rapidly-growing metropolitan area. Using the traffic flow map and cameras, our Traffic Management Center staff monitor traffic every day of the week, continually looking for freeway accidents or other incidents. When a problem arises, we can immediately verify the incident with the cameras and report it to emergency response personnel, area media and our radio dispatch units. We also alert drivers by activating variable message signs, which are large, illuminated signs that span the freeways, as well as record highway advisory radio warnings that are broadcast to AM 530 on your car radio. Using the cameras to verify incidents helps ensure that these messages stay accurate and timely during constantly changing freeway conditions.

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What if my question is still not answered?

You can ask us questions or provide feedback about the Seattle area site by sending us an email. Other WSDOT-related topics should be sent to WSDOT web feedback.

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