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Network Data Collection

The MPO and TxDOT-District typically perform the initial step in network development by identifying which facilities should comprise the base year roadway network. Generally, all facilities functionally classified as a collector, or higher functional classification, are included in the roadway description provided by both agencies. Primary network attributes are then inventoried by the MPO to be included in the network database; these attributes include facility type, number of lanes, and whether the roadway was a divided or undivided facility. Physical and operating characteristics including roadway length, number of lanes, and median access type (divided or undivided) as well as operational characteristics such as daily speed, daily capacity, average weekday traffic count and direction (one-way/two-way) are also developed for the network database. Apart from link distance, physical characteristics are primarily provided by the MPO and operational characteristics are developed by TPP.

Based on the roadway inventory, the MPO provides the following network attributes:

• Number of lanes - the total number of lanes in both directions; however, the total does not include continuous left turn lanes.

• Posted speed limit - the facility’s posted speed limit

• Direction - the specification of whether a facility is a one-way or two-way facility

• Median Access Type - Divided, undivided or continuous left turn facility

• Functional classification and facility types - each network link is identified by a functional classification and facility type. Functional classification is a grouping of streets and highways into a set of classes or systems based on the type of highway service they provide. Generally streets and highways provide two types of services: traffic mobility or land access. Every class of roadway serves as a collecting or distributing facility for the next higher class of roadway. Freeways and arterials primarily facilitate traffic movement and mobility whereas collectors and local streets primarily provide access to property and are associated with lower speeds. Local roads are generally not included in the network. Typical functional classifications include: Interstate highway, freeway, expressway, major and minor arterial, collector, local, ramp, and frontage road. Facility types are more precise classifications to properly code representative operational characteristics; for example, divided and undivided principal arterials.

TPP develops additional roadway descriptive characteristics for each network link:

• Distance - the length of each network link is required in determining link travel time. TPP uses the distance automatically calculated by the modeling software (TransCAD).

• Area types - Area types designate physical boundaries where operational characteristics are assumed to change due to land use densities and the resulting impact on travel propensities.

• Capacity - the traffic assignment process uses roadway link speeds and capacity. TPP develops capacity look-up tables for network roadways based on area type and facility type. The capacity assigned to each network link is cross-classified by facility type and area type. The capacities typically used are based on a service volume at level of service E and the Transportation Research Board Highway Capacity Manual guidelines.

• Speed - TPP develops speed look-up tables for network roadways based on area type and facility type and is typically an average 24 hour operational speed. The trip distribution process uses network link distance and speed to produce an estimate of link travel time, from which minimum path routes between all traffic analysis zones are developed.