This project could not achieve it's goal without the help and support extended by Dr. Douglas Ramsey, Dept. of Geography and Earth Resources, USU. Regarding the data of signs and signals of Logan, the help and cooperation of Mr. Ross Wilson, City Engineer, City of Logan, Utah, is also acknowledged.
Some thousands of student, faculty, employee and other people make trips to and from Utah State University (USU) everyday throughout the year. Different people travel through different routes to the university depending on their origin while driving their own vehicle. In this study it was decided to find the minimum travel time path from diffent points in logan. For the study 15 different origin points in Logan and a destination point in USU were selected. Between these origin and destination minimum time routes were analyzed.
At the beginning it was decided to work on the entire city of logan. But considering the time constraint and the volume of data required it was decided to keep the study area within the boundary bounded by Main street, 1000 North, 1200 East, U.S.Highway 89, Boulevard and Center street.
For the desired analyses to find the minimum impedance route the data required were :
1) Street maps of Logan as the base map,
2) Location of STOP, YIELD and Signals in Logan and the
cycle time of the signals,
3) Speed-limit data for all the roads of the study area.
The street base map of Logan was clipped out of the Utah Geographic Database file named "utahrds" that was available in computer database. First the quadrangles that had the Cache County road network were separated from 'utahrds' and was named as 'uthrds_ca'. Then from 'utahrds_ca', the road network map of Logan was clipped off. The sign and signal location data were collected from Department of Engineering, City of Logan. Signal cycle timing data were also collected from the same office. The speed limits on different roads were collected through road survey.
The turn impedance regarding STOP, YIELD and NO-STOP has no established values. So, these values were selected through a careful subjective judgement. The impedance values that were selected are given below:
(a) Turn impedance in the direction of "STOP" sign:
Left turn = 0.50 min.
Straight = 0.25 min.
Right turn = 0.17 min.
U-turn = -1.00 min.
(b) Turn impedance in the direction of "No Stop":
Left turn = 0.25 min.
Straight = 0.00 min.
Right turn = 0.10 min.
U-turn = -1.00 min.
(c) Turn impedance for signalized intersection:
Left turn = 1.00 min.
Straight = 0.50 min.
Right turn = 0.17 min.
U-turn = -1.00 min.
It was decided not to allow any U-turn. That is why we used negative value of -1.00 for u-turns.
The clipped road coverage (logan_rds) of Logan was not directly usable. There were no node attribute table associated with the coverage. The node attribute table (logan_rds.nat) was created. But lots of pseudo nodes were observed after the 'NAT' file was created. The original data source (utahrds) was quite good. But as it was a road network map of entire state of Utah, all details were not correct. Extensive checking for all nodes and arcs were done to find whether the arcs at the intersection meet at where they were supposed to meet. After the coverage was made usable through eliminating pseudo nodes, unwanted dangling nodes and unwanted arcs, build and renode commands were executed to make the node numbers sequential.
To find the minimum time (impedance) route between two points (nodes), Pathfinding option of network analysis in 'Arc/Info' was used. There were two different ways to find minimum impedance route in 'pathfinding'. One was the 'PATH' option which finds the minimum impedance path to reach a series of stops in user defined order. Another option 'TOUR' which is similar to PATH , except that TOUR determines the order in which stops are visited. Though any one of the options could be used for the analyses, it was decided to use 'TOUR' as there were not any intermediate stops between origin and destination. The steps followed in preparing data files for 'TOUR' were as follows :
- preparing the network coverage AAT, - assigning link impedance values, - creating a turntable, - assigning turn impedance values, - creating stops files, - assigning values in stops files.
As the project was to find the minimum time route from 15 locations to USU, the impedance for the network analysis was time (calculated in minutes). The distances or the length of the roads in the "AAT" was in meters. In AAT, four new items were added. An item "Name" was added and all the street name of Logan were assigned under that item. The "Name" item was later used to find the "directions" of the minimum time routes. Other three items were required to find link impedance values of each arc. An item "Speed-limit" was added to assign speed limit values of each arc. The speed limit values were converted from miles per hour to meters per minute to avoid any calculation difficulty and to keep the units consistent. The third and fourth item were added to AAT to assign from-to-impedance and to-from-impedance of each arc respectively.
The link impedance values are the from-to-impedance and to-from-impedance. These impedances were assigned to item "f-t-imp" and "t-f-imp" through the arcplot command "Calculate" where target_item was equal to 'length / Speed-limit'.
The next step was to create a "turntable" for the coverage. The turntable was created by turntable command in Arc. A record in a new turntable would look like the following:
NODE# = 2 ARC1# = 61 ARC2# = 2 AZIMUTH = 1.486 ANGLE = 57.615 ARC1-ID =1,742 ARC2-ID =1,670
9 or 16 records were created in turntable for each node with 3 or 4 arcs at the node, respectively. Then an item called "turnimp" was added to turntable "Logan.trn" to assign turn impedance values. After declaring the coverage as "Netcover" in Arcplot turn impedance values for left, straight, right and U-turn were assigned to the item "turnimp" through "setturn" command. But these impedance values were common for all intersections irrespective of signs and signals. But it was realized that turn impedance values for directions with signs and signals and without signs and signals could not be same. Thus different values for different situations were decided and those values were given under the heading "Data Sources". It was decided not to allow any U-turn. That is why we used negative value of -1.0 for U-turns.
To put different turn impedance values for more than 66 intersections with "STOP" sign and 18 signalized intersections, the main work was with the "turntable" of the coverage in INFO. The following figure shows the "STOPS & SIGNALS" of the study area.
First, in Arcedit, the coverage map was brought up with arcs, nodes and ids to see which arcs are related to which node. Then in INFO, the ".TRN" file was selected. And then a particular node with NODE# was reselected. But the problem was NODE# and Node-ID's were not the same. But in Arcedit only Node-ID could be displayed instead of NODE#. As a result, a print out of ".NAT" file helped to figure out the NODE# of the corresponding NODE-ID to select the node in INFO.
The record for the selected node could be listed in Arcedit to set the NODE# of the corresponding NODE-ID. However, the print-out of "NAT" seemed more convenient than shifting between Arcedit and INFO.
After selecting the desired node, with a node-id on the screen, by NODE# in INFO, we reselected the turn records sometimes by 'ANGLE', sometimes by ARC1-ID, sometimes by ARC2-ID, and sometimes by 'TURNIMP' depending on the ease of selection. Then we changed the impedance values by "CALC" command.
We also added the speed limit value for all the arcs of the coverage through Arcedit. In Arcedit, we selected the arcs with same speed limit by command "SEL MANY". Then we added the speed limit values by "CALC" command. All the speed limits were given in meter per minute. The unit of speed was selected as meter per minute because the arc lengths were in meters and the turn impedance values were in minutes.
The next step was to create "Stops" file. Stops files were created in "INFO" for 15 routes. Stops files determine the sequence of stops to be visited. The origin and destination intersections for 15 routes were as follows.
Route Origin Destination number intersections intersections
1 Main st, 1000N 800E, 700N 2 Main st, 700N 800E, 700N 3 Main st, 400N 800E, 700N 4 Main st, 200N 800E, 700N 5 Main st, Center st 800E, 700N 6 200E, 1000N 800E, 700N 7 200E, 800N 800E, 700N 8 200E, 300N 800E, 700N 9 200E, Center st 800E, 700N 10 300E, 600N 800E, 700N 11 400E, 800N 800E, 700N 12 400E, 400N 800E, 700N 13 600E, 1000N 800E, 700N 14 600E, 300N 800E, 700N 15 1200E, 1000N 800E, 700N
The following figure shows all the origin and destination points.
Creation of stops files and addition of item values were done in INFO by DEFINE and ADD commands respectively. A record in a stops file would look like the following:
LOGAN-ID = 68 IN_ORDER = 1 ROUTE_ID = 1 STOP_IMP = 1 TRANSFER = 0 CUMUL_TRANS = 0.00000 CUMUL_IMP = 1.00000 OUT_ORDER = 1
The main problem was to add turn impedance values of diffenent situation of intersections. The 'setturn' command allowed only single value for left, right, straight and u-turns. But turn impedance values can not be the same for STOP, NO-STOP, YIELD or Signals. So different values for different sign and signal situation were added to '.TRN' file. Some editing of the coverage were required after addition of turn impedance values. Some arcs were deleted and resulting pseudo nodes were taken off. As the "build' and renode' commands were executed, NAT, AAT files were updated but not the 'TRN' file. The 'turntable' command updated the "trn" file except the added item 'turnimp'. All the 'turnimp' values became zero. As another copy of the coverage was created with turntable before editing the original coverage, the turn impedance values from the edited coverage were added to the copied "TRN" file through "RELATE".
As the first attempt a stops file was created for all origin and destination nodes with different route ids. After executing the "TOUR" command, when the routelines were plotted in the map, all the minimum impedance routes from each origin were shown in one color. This happened because of single stops file. When a coverage is declared as "NETCOVER" for pathfinding, the whole stops file was used for that path declaration. Though the routes are analyzed according to route ids in the stops file, the "ROUTELINES" command shows all the routes in one color due to single path declaration in "NETCOVER" command which uses single stops file and creates single route attribute table. This problem was handled by creating different stops files for different routes. 15 different route maps were created in Arcplot to avoid overlapping which was obvious if plotted in one map.
Following are the directions of "route 1":
Starting at 1000 N Travel for 2.269 units, turn right onto 800 E Travel for 0.751 units, End of path Total path length is 3.021 units
Following are the directions of "route 2":
Starting at 700 N Travel for 1.853 units, turn left onto 700 E Travel for 0.262 units, turn right onto 800 N Travel for 0.252 units, turn right onto 800 E Travel for 0.262 units, End of path Total path length is 2.629 units
Following are the directions of "route 3":
Starting at 400 N Travel for 1.521 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 2.705 units
Following are the directions of "route 4":
Starting at 200 N Travel for 1.142 units, turn right onto BOULEVARD Travel for 0.129 units, turn back onto BOULEVARD Travel for 0.696 units, turn left onto 600 E Travel for 0.180 units, turn right onto 400 N Travel for 0.201 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 3.532 units
Following are the directions of "route 5":
Starting at Center St. Travel for 0.530 units, turn left onto BOULEVARD Travel for 1.406 units, turn left onto 600 E Travel for 0.180 units, turn right onto 400 N Travel for 0.201 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 3.502 units
Following are the directions of "route 6":
Starting at 1000 N Travel for 1.581 units, turn right onto 800 E Travel for 0.751 units, End of path Total path length is 2.332 units
Following are the directions of "route 7":
Starting at 800 N Travel for 1.572 units, turn right onto 800 E Travel for 0.262 units, End of path Total path length is 1.834 units
Following are the directions of "route 8":
Starting at 200 E Travel for 0.246 units, turn right onto 400 N Travel for 1.002 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 2.432 units
Following are the directions of "route 9":
Starting at BOULEVARD Travel for 1.406 units, turn left onto 600 E Travel for 0.180 units, turn right onto 400 N Travel for 0.201 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 2.972 units
Following are the directions of "route 10":
Starting at 600 N Travel for 1.064 units, turn left onto 700 E Travel for 0.527 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 1.714 units
Following are the directions of "route 11":
Starting at 800 N Travel for 1.054 units, turn right onto 800 E Travel for 0.262 units, End of path Total path length is 1.316 units
Following are the directions of "route 12":
Starting at 400 N Travel for 0.604 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 1.788 units
Following are the directions of "route 13":
Starting at 1000 N Travel for 0.555 units, turn right onto 800 E Travel for 0.751 units, End of path Total path length is 1.306 units
Following are the directions of "route 14":
Starting at 600 E Travel for 0.180 units, turn right onto 400 N Travel for 0.201 units, turn left onto 700 E Travel for 1.061 units, turn right onto 700 N Travel for 0.123 units, End of path Total path length is 1.565 units
Following are the directions of "route 15":
Starting at 1200 E Travel for 0.670 units, turn right onto 700 N Travel for 1.531 units, End of path Total path length is 2.201 units
Though the result may seem very simple, the whole process was very time consuming. Right from the beginning of data collection to the end of the analysis intensive care and deep concentration were required. A small mistake could jeopardize the entire process. The spatial database has to be cleaned according to requirement before any attribute is added to turntable.
Arc/Info allows for link impedance, turn impedance, and stop impedance in the pathfingding analysis. But it does not differentiate among signal, STOP, YIELD, and NO-STOP situation. That is why adding turn impedance values were very time-consuming. Arc/Info also does not allow adding speed limit value according to road classification or special zones like school or hospital zone.
Due to time constraint all possible techniques of Arc/Info could not be explored. If more time were allowed, a bigger study area could be taken and a better analysis could be done with possible short AML programs for different analysis and display.
Another simultaneous analysis which had been in mind, could not be done due to time constraint, was to find minimum impedance bus routes.
With the experience of this project, it can be concluded that Arc/Info has a strong capability with few limitations to find the minimum impedance travel rotes and more extensive work can be done if more time is available.
1. 1994. Understanding GIS, The ARC/INFO Method. ESRI.
2. ARC Command References, Ver.6.0, ESRI.
3. ARCEDIT Command References, Ver.6.0, ESRI.
4. ARCPLOT Command References, Ver.6.0, ESRI.
5. 1991. INFO Reference Manual, Henco Software Inc.
6. ARC/INFO on-line help.
7. Winter,1995. Logan Transit District (LTD) map.
8. May,1993. Pathfinding and Allocation, Training course
class materials, ESRI.