Frequently Asked Questions

Here are answers to some FAQ's. Keep them handy.

A: Newer versions  of QRS II do not recognize the name AddTrips.txt.  Your AddTrips.txt should be renamed AddVTrips.txt or AddPTrips.txt depending upon whether it contains vehicle trips or person trips.

A: GNE displays the networks exactly as they are originally drawn, with the upper-left corner of the underlaid network's Sketch Pad aligning with the upper-left corner of the top network's Sketch Pad.  White space is sometimes needed on one or both networks to get them aligned.  The best method to establish the exact needed white space is to place one node on each network somewhere above and to the left of all the rest of the nodes.  Both nodes should be at the same x, y coordinates.  Then "trim" both networks and they will align properly.  Of course, both networks must be drawn to the same scale.  Another method is to set the "extents" of the networks so they have the same top-left (northwest) corner.

A:  Yes, but not on a single network view.  It is possible to label a link on both the original network and on a view of this network.  Select positions for the two sets of labels so that they do not overlap.  Then use the view as an underlay.  Use different fonts or colors to distinguish the source of the labels.

A: Yes. GNE maintains links and nodes in a specific order depending upon their geographical locations. The smallest node numbers are in the upper left corner of the network while the largest node numbers are in the lower right corner of the network. The order of links is controlled by the order of their A-end nodes.

A: The easiest method for finding the node serial number is to bring the network into Advanced GNE and label all nodes with their serial numbers (Go to: View, Labels, Node Labels, First; then select Serial).

A: The serial number of a link is the same as the link's order in the DTA file. If a link is named, its name and internal serial number will appear within the report file LINKLABL.TXT. If the link is not named, it is still possible to find the link with GNE program FINDLINK.PGM. See Appendix F for a description of FINDLINK.PGM.

A: At a traffic controlled intersection, QRS II has detected three or more approaches with the same through traffic code. The cause is probably a 0 (zero, first position) as the through traffic code at an oblique intersection approach. Find the intersection (see above discussion of finding links) and explicitly set the through traffic codes.

A:  GNE wants to guarantee that you have correct approach codes, and will prevent you from leaving a link that does not. The error might be that you have not added in the new code in the fourth position, which is the code for functional class. We suggest a code of "O" ("Oh", not "zero") in most cases. The other possibility is that your link originally had an incorrect code, and GNE now does not like it.  See the QRS II Reference Manual for a discussion of codes for functional class; also read the sections (see index) on approach codes.  The problem can also occur with incorrect codes on centroids.  Newer application schemas have suggested default codes embedded in the attribute name.

A: The cause is probably a centroid or external station that is disconnected from the network or too far away from all other centroids. Perhaps the scale is set improperly. Ask for temporary files and view HTIMES.TMP in Notepad. If you see many 2400's (largest possible travel time), then there is a disconnection somewhere. Note: the rows and columns of HTIMES.TMP are in the same order as the centroids in NODELABL.TXT.

A: The calculated delays are much too large. Check all parameters related to delay, including the hour averaging factor for multihour assignments. Look for capacity values that are too small.  One or more of these will be seriously in error.

A: You have a user-supplied trip table and traffic-controlled intersections. You need to reset VOLHOUR.TXT (older versions) or VOLHOURU.TXT (newest version) each time you switch time periods.  VOLHOURU.TXT is not needed if "Assume Hourly Volumes" is checked or you are running a single hour forecast or a dynamic traffic assignment.

A: VOLHOURU.TXT is an input file; VOLHOUR.TXT is an output file. QRS II will create a new VOLHOUR.TXT when there is no user-supplied trip table. Run your network, omitting the trip table and with 0 equilibrium iterations. Then copy the resulting VOLHOUR.TXT to VOLHOURU.TXT, to serve as input.

A: You must tell QRS II to expect ADDVTRIPS.TXT on the Add Files dialog box. Also, ADDVTRIPS.TXT must be in the same folder as PARAM.TXT (and other active parameter files).

A: Usually, this error means that you missed a link when entering the approach codes. The approach codes probably consist of blanks and are nearly impossible to detect by displaying them on the Sketch Pad as labels. Use Extract in GNE to spill link information to a text file. Use a format of BXY1 (name, x-coordinate, y-coordinate, approach codes). You can quickly locate the offending link by scanning the resulting text file in Notepad.

A: The LOS C definition of "practical capacity" extends back to the 1950's, but it is inconsistent with our current knowledge of traffic flow. If you are using traffic controlled intersections in QRS II you must use a LOS E definition. This implies capacities on urban streets of about 1600-1700 vphpl (vehicles per hour per lane) and on freeways of about 1800-2000 vphpl, depending upon prevailing conditions. Be sure to change the volume to capacity multiplier (on the Assignment BPR Parameters dialog box) to about 0.8. You should also consider setting the opposite direction factor to 0.4 to obtain better delay values on two-lane roads.

A: Version 7 of QRS II behaves somewhat differently than earlier versions.  In earlier versions of QRS II, the lane geometry and sign code was ignored for traffic coming from the stem of the T.  The reason for this was that QRS II could not detect the existence "left" turns anyway and there could be at most one exclusive right lane.  The capacity of the approach was dictated by the through lanes only.  In version 7, QRS II tries to use the approach codes, if possible. Again, with no opposing traffic, left turns will be treated as "other" movements and lumped in with throughs.  Right turning lanes, however, will add capacity.  For example, an approach from the stem of a T with one true left and one right lane should be coded as having a single through lane (capacity of about 1800 vphg) and one right lane.  In some instances it may be desirable for left turning lanes to be explicitly shown.  For these lanes to have any effect, the network must suggest the possibility of opposing traffic by having a stub link on the other side of the intersection.  In any event, it is recommended that the behavior of the intersection be checked in Signal.txt.

A:   Right turns can be prohibited by putting the turning movement in AddPenalty.txt with a high penalty value.  See page 141 of the QRS II Reference manual.

Q: How do I prohibit left turns from a one-way link?

A: Split the approach link into a pair of links: a one-way link and a two-way no left turn link. The one-way link prohibits wrong-way traffic for the pair, and the two-way no left turn link prohibits left turns for the pair.

Q: How can I utilize digitizer support?

A: Most people prefer to scan the document, create a JPEG file, and place the image behind the network as a background.  However, it is still possible to use a digitizer with GNE.  One of the great side-benefits of Windows is digitizer support. Each digitizer is somewhat different, so it is difficult to give general instructions. I once used a Summagraphics digitizer, so my comments relate to that particular brand. You should contact your digitizer manufacturer for their latest set of Windows drivers. The digitizer replaces the mouse. Use Windows Setup to make the switch. My digitizer comes with a program to set the behavior of the digitizer. Among other things, it permits the digitizer to report absolute coordinates, work within an area that matches a window on the screen, and to scale the drawing. Use the digitizer to control the node plot cursor. You can digitize the original locations of nodes or you can move nodes. Once nodes have been positioned, you can add links as usual.

A: Before starting the plot, check that you have:

1. Set the paper size on the plotter;
2. Set the Windows driver, through the Windows control panel;
3. Set the scale on the Page Setup dialog box  -- you may need to enlarge it to plot;
4. Set the margins, measuring from the top left of the paper;
5. Shift the image if necessary -- to shift upward or to the left, use a negative number.

A: Yes, although you will be able to print out only what is on the screen, rather than the entire network if its size exceeds the screen size. To carry out a "screen dump," click anywhere on the Sketch Pad. Then press Alt Print Screen. This will put the screen image into the Clipboard. Then paste the network into Paint (or some other graphics package) to edit and print the image.

A: To get boarding and alighting information at metro stations, designate the transfer point to be a "station". Embed an "<S>" within the name of the node. You will then get a station report. Unfortunately, QRS II will not produce station reports at regular stops.  (You can turn a stop into a transfer point for this purpose by adding a stub link with a different route number.)

A: If your network is large, you should avoid ladders in order to keep your network small. Remove any transfer points that are not totally necessary. Consider eliminating some points of access (centroid connectors) to reduce network size.

 

For a ladder to work properly (in theory), all routes must connect to the same node while requiring exactly one transfer. That single node should be a transfer point. When this is a point of access, the centroid connector should connect to the transfer point. When done this way, riders will have equal access to all routes at that location. The rungs of the ladder will have the same route numbers as the rails of the ladder.

With a very large network, you may want to restrict transfers between selected routes and restrict access to certain other routes. Or consider two or more centroid connectors at a single location with each centroid connector being attached to a unique set of routes. However, you must use your own judgment on which of the transfers should be restricted by these methods.

Ladders can be avoided entirely with the use of AddRoute.txt.  See the QRS II Reference manual for details.

A: I recommend that you use subarea focusing to handle small areas. Use tiny zones in the subarea and large zones outside. Use a fine street system in the subarea and a coarse one outside. Use less detail the farther you get from your subarea. This is essentially the method described in NCHRP Report #187. You might also want to review NCHRP Report #255 for additional hints on travel forecasting.

A: With versions 5.1 or earlier, you must use two parallel one-way links.  Versions 6 and later allows different capacities in different directions with the "detailed" and "dynamic" schemas.

A: You can plot or print part of a network by taking a portion of an existing network, consisting of an irregular subarea. Draw a polygon around the subarea, choosing existing intersections as vertices and an existing centroid as the owner. Perform a polygon search, hiding all elements outside the polygon. Move all visible elements to a unique layer, then delete all elements on the remaining layers. The remaining links and node within the desired subarea can be saved to a new file. Each of these steps requires only a few mouse clicks. Then print. See also the chapter on Polygons in the Reference Manual.  Of course, it is always possible to define the image shift and plot margins to print a small rectangular area of your network.

A: You have told QRS II to hold productions constant on the Trip Productions dialog box.  QRS II will only use attractions at centroids to balance; attractions at external stations are never adjusted.  However, QRS II cannot find any attractions at the centroids.  You should tell QRS II to hold attractions constant for this purpose or add a small trip attraction value (e.g., 0.0001) to one of your centroids.

A. Generally, the best way to create a GNE network from text file data is to use the HNET Conversion Utility that is provided with QRS II versions 5 to 7. This module of QRS II Tools will set the approach codes to a minimally valid set, namely "0S3O". If the network was created with some other program, then it is possible to set the approach codes with GNE’s Calculate. It should be noted that GNE’s Calculate cannot operate on entities that are undefined. Also, the network must be based on a valid application schema. It may be helpful to observe that one-way links have an entity type of 1 and two-way links have an entity type of 2.

Here is the procedure for using Calculate to set the approach codes on one-way and two-way street links.

1. On the Link Line Ranges tab set the "begin" and "end" values to 1 for both two-way street links and one-way street links.

2. On the Relations tab, in the first line enter the following equation:

 a1 = ‘0S3O’

or any other valid set of approach codes.
3. Press the Start button. Open several links to assure that the program has worked properly.