What is traffic data?

Traffic data provides information about how travel speeds on specific road segments change over time. It is important in network analysis because traffic affects travel times, which in turn affect results. If you are planning a route from one location to another and don't account for traffic, your expected travel and arrival times could be far from accurate. Moreover, you might miss routing opportunities that save time by avoiding the slower, more congested roads.

The following two graphics demonstrate how the quickest route can change at different times of the day due to changes in the prevailing traffic patterns.

At 8:00 a.m., commuter traffic to the city is heavy, but traffic to the suburbs is light; therefore, the quickest route from the city to the suburbs at this time is along the divided highway.

At 5:00 p.m., commuters are driving home from the city, which increases the travel time in that direction. Travel has become slow enough on the divided highway that the serpentine alternative now offers a quicker route to the suburbs.

With the ArcGIS Network Analyst extension, you can store travel speeds in the network dataset using two different models: historical and live traffic. This allows you to visualize traffic on a map and perform network analysis given current or typical traffic speeds.

Historical traffic

The historical traffic model is based on the idea that travel speeds follow a week-long pattern. Thus, at 8:00 a.m. on Monday of one week, the travel speeds of a given road segment are expected to be similar to those at 8:00 a.m. on Monday of another week. Since the duration of the pattern is one week, the speeds aren't necessarily expected to be similar among different days of the same week. That is, congestion and travel speeds may differ significantly for the same road segment, at the same time of day, but on different days of the week. For example, travel speeds on Main Street at 8:30 a.m., Sunday, could be much faster than at 8:30 a.m., Monday.

The expected speeds are usually determined by averaging multiple observations over some time span, such as a year. This process is described more in the Historical traffic topic.

As long as your data is reliable, performing a network analysis using the historical traffic model will tend to return much more accurate results than those returned using a single travel cost irrespective of time or day of the week.

Live traffic

The live traffic model fills in where the historical traffic model falls short: accounting for current traffic conditions. As you know from the last section, historical traffic is based on average travel times. However, current travel times can deviate considerably from the norm. For instance, events that draw large crowds can cause traffic to slow, accidents can temporarily halt traffic, and holidays can slow traffic or change where traffic congestion occurs. When you solve a network analysis whose results will be implemented immediately or almost immediately, using live traffic will tend to improve results, even over those created with historical traffic.

With the live traffic model, data providers measure current speeds using a number of sources, such as GPS receivers in vehicles and speed sensors on roads. Network Analyst uses geoprocessing tools to connect to the data providers over the Internet, download the live travel speeds, and feed them into the network dataset for both visualization and network analysis purposes.

Predictive traffic in live traffic

Of course, a single snapshot of live traffic used for network analysis purposes would not be entirely helpful because traversing a network takes time, which means an analysis extends into the future, and traffic patterns change with time. This is one reason data providers also process live traffic data to make travel-speed predictions of a given depth, such as for the next 12 hours. This way, a route, service area, or other network analysis can start its network traversal from one point using live traffic, then switch to predicted speeds as the traversal extends outward.

As the network analysis queries edges for travel speeds that span farther into the future, speed projections become inherently less reliable—that is, the probability of other events occurring and affecting traffic increases. Once the prediction depth is reached, Network Analyst falls back to historical traffic speeds.

Note:

To help you decide on which Help topics to read, consider that, although historical traffic can be configured without live traffic, live traffic must be configured along with historical traffic.

Learn more about live traffic

Visualizing traffic and performing network analysis using traffic

Traffic, especially live traffic, is used for two main reasons: to see traffic conditions and to perform time-dependent network analysis. With the ArcGIS Network Analyst extension, you can do both. The subsections below outline how this is accomplished.

Visualizing traffic

There are two ways to visualize traffic. The most common way is to add an ArcGIS Online traffic map service to your map documents or websites. The other way is to add a traffic-enabled network dataset to your map document.

Visualizing traffic on your own network dataset requires the general steps listed below.

1. Configure historical traffic on a network dataset.
2. Optionally, configure live traffic in the network dataset. You can do this by pointing the network dataset to a directory where traffic data will be downloaded to, then running the Update Traffic Data geoprocessing tool to download traffic data to that directory. The Live Traffic topic provides more conceptual information to help you retrieve live traffic data from a data vendor.
3. Add the network dataset as a layer to a new ArcMap document. By default, traffic conditions for the current time and date are displayed.
4. To see traffic conditions for a different time and date, enable time on the map with the time slider, which is accessed from ArcMap's Tools toolbar, then move the slider. Disable time on the map to see current traffic conditions again.

Keep in mind that if you don't have a traffic-enabled network dataset, you can view traffic using an ArcGIS Online traffic map service instead.

Learn more about visualizing traffic

Visualizing live traffic incidents

Traffic incidents tend to reduce travel speeds. They include events like road construction and traffic accidents. A traffic map can be enhanced by adding traffic incidents because they help explain why travel speeds are slower in certain areas. ArcGIS Online provides map and feature services of traffic incidents that you can subscribe to and add to your own maps. If you want to use a different data feed, follow the steps below as a general guideline for downloading and viewing traffic incidents.

1. Download the latest traffic incident data using the Update Traffic Incidents geoprocessing tool. This creates or updates a point feature class.
2. Add the feature class to the map.
3. Symbolize and, optionally, label the points.

Tip:

ArcGIS Online provides map and feature services of traffic incidents, which you can subscribe to and access without running the Update Traffic Incidents tool.

Note:

Traffic incidents are for visualization purposes only, which means the network dataset can't use them, at least not directly, to adjust travel costs. To use them indirectly, you can create barriers from incidents. But keep in mind this is only useful if you have historical but not live traffic. Live traffic data already accounts for the effect incidents have on travel speeds, and data vendors preprocess incidents to make better travel-speed projections. The live and projected traffic data is all the network dataset really needs.

Performing network analysis using live traffic

All of the Network Analyst solvers support traffic, which means you can perform time-dependent network analyses for generating routes and service areas, performing location–allocation, and so on.

An overview of what is required for time-dependent analysis is provided in Network analysis using traffic data. More detailed information about setting up traffic with the various network analysis layers is provided in their respective overview topics.