APPENDIX A: GIS METHODS AND WIM STATIONS

This appendix summarizes the procedure of generating the WIM station shapefile for the state of North Carolina. The weigh-in-motion (WIM) systems provide massive amounts of valuable data related to trucks and general traffic. There are about 60 WIM stations located along Interstates, US, State, and NC routes.

Quality Assurance (QA) procedures are conducted regularly to point out problems at the WIM site and help maintain the system throughout the site design life. The need for quality assurance prompted the development of software programs that are used to validate data and point to problems occurring at the WIM site. These programs include the Long Term Pavement Performance (LTPP) QA software, the Vehicle Travel Information System (VTRIS) and individual state software.

The data used for this project uses VTRIS database for all WIM locations throughout North Carolina. The VTRIS software replaces the Truck Weight Software and uses the standards, formats, and methods specified by the Traffic Monitoring Guide (TMG). VTRIS is a recommended, but not required, method to submit data to the Federal Highway Administration (FHWA) in a uniform format. The software validates, summarizes, and generates reports on vehicle travel characteristics by lane and by direction in the TMG format.

The VTRIS software develops and maintains a permanent database of the WIM data. Traffic data is available from 1997-2002 for the state of North Carolina. It can be viewed using the W-2 table from

http://apps.fhwa.dot.gov/vtris/vtris.aspx. The data are validated by VTRIS before inclusion into the VTRIS maintained database. The validation process can be adjusted for each station’s site characteristics and user defined parameters for axle spacing and axle weights. Errors detected by the software can be viewed to determine the type of error and whether or not to include the record in the database. The software also converts the WIM data to metric units, thus complying with the FHWA Metric Conversion Plan.

The data obtained from 1997-2002 in W-2 table is grouped by station number (i.e. SHRP#) and converted to excel format for analysis. Growth factor for each station for total traffic, duals, TTST, and total trucks are calculated. This table is used to join with the new WIM shapefile.

Data

Shapefiles used for the editing

- DOT roads layer dotroads.shp

- National Highway system layer nhs98all.shp - WIM stations layer wim37.shp

- Counties layer all_counties.shp MS Excel file

- VTRIS stations.xls

Creating the Shapefile and Editing Open ArcMap

- Open the Editor toolbar (View/Toolbars/Editor)

- In the Editor toolbar click Editor/Start Editing. You may be given an option of a folder or database from which you want to edit data. Choose the folder where s37wim.shp lies and click OK.

- In the Editor toolbar click Create New Feature in the Task window then scroll to and select the shapefile to edit within the Target window.

- In the Editor toolbar click the Create New Feature icon (it looks like a short pencil).

- Before adding a new feature snapping is switched on. The vertex of the s37wim is to be snapped with the edge of the dotroads layer. A snapping tolerance of 20 is given for maximum accuracy. The checkbox for showing snapping tips is also checked.

- The location description of WIM stations is used to create a new feature on the s37wim layer. Stations are located on the road network and are referenced to specific distance from major roads, mileposts, exits, or major towns.

- First the route on which the stations are located is selected from the dotroads layer using selection by attributes.

- The reference for the WIM station is used to determine the exact location of the station. In cases where distance from an intersection route is given, the measuring tool is used to locate the new station.

- A new station is added on using add new feature tool.

- Since the wim37.shp is a point layer, new point features are created and a row for the new feature is added in the attribute table.

- After the new feature is added to the s37wim shapefile, click on the attribute table on the editor toolbar and add the station ID number to the new feature.

- When finished editing, click Editor/Stop Editing and click Yes in the Save window. Joining Two Tables

The WIM data collected from VTRIS website is analyzed and saved as a excel file named VTRIS stations.xls. The file is saved to a dbaseIV format in order to be read into ArcMap. In order to perform a table join, both tables should have one column similar. The attribute table for s37wim.shp contains a column for station ID named STTNID and the VTRIS station.dbf has SHRP# column that matches the STTNID column.

- In ArcMap Add Data and select the file (you won't see it on ArcMap unless you select the Source tab in the Table of Contents window)

- Right click s37wim layer and select ‘Join’ to join the dbase table.

- The join is based on the field STTNID in the wim layer and SHRP# field in the dbase table. - The two tables are joined together and the attribute table in the wim layer displays the joined

output.

- In order to associate the join into the shapefile, the wim layer is exported and saved as a new shapefile.

This shapefile can be used to locate statewide weigh-in-motion stations. Once the station number is identified, the data corresponding to that particular station can be obtained from the on-line VTRIS spreadsheet. The data include the VTRIS data:

- station characteristics; - station location;

- annual traffic classification counts by lane and direction; - axle spacings;

- vehicle weights, etc.;

and additional analysis is associated with the spreadsheet:

- average growth factor for the range of the traffic data including AADT and composite vehicle classes corresponding to the NCDOT truck classifications Dual and TTST;

- growth factor ratios;

- growth factors commonly used with the NCDOT Traffic Forecasting Utility (Trend Program);

- statistical checks and guidelines based on QQ plots and confidence intervals to help the analysis validate and choose appropriate growth factors.

The analyst can incorporate the resulting VTRIS shapefile and database with local, regional, statewide and national databases for NCDOT project locations, economic trends, land use, building permits, traffic generators, planned industrial sites, and the like to provide additional insight to the traffic trends, growth factors and traffic forecasts being developed.