Because road construction can be costly in both an economic and environmental sense, proposed roads should be analyzed to assess how well they would satisfy desired benefits. One desirable characteristic of a new roadway is a reduction in the necessary travel distance to frequently visited locations for the maximum number of people. I used a Geographic Information System to study how proposed roads, as shown in the most recent amendment of the Northfield Comprehensive Plan, would affect on-road travel distances in and around Northfield. That is, for each of a series of destinations around Northfield, I used GIS software to find the on-road travel distance from any other location on the map to the destination. By doing this for the existing road network, along with a road network to which the proposed roads have been added, the effect of the proposed roads on travel distance can be determined. I used four destinations near the edge of town (one each on Hwy. 3 north, Hwy. 3 south, Hwy. 19 west, and Hwy. 19 east) and one in the center of town, at Bridge Square. If all of the proposed roads were to be built, the maximum reduction in on-road travel distance would be less than one mile, a change that translates into a travel time difference of less than 2.5 minutes, assuming a speed of 25 mph. Furthermore, because the proposed roads essentially form a "ring road" around the outskirts of town, the travel distances for traffic to or from the downtown are essentially unaffected. Because the map of existing streets has not been checked in extreme detail for accuracy, this analysis should be considered preliminary. However, this study does demonstrate the potential for the use of GIS in transportation analysis and planning.
When selecting a route or building a road between two points, the most logical pathway is the one that minimizes the travel distance or, in some cases (mountainous terrain, for example), the one that requires the least travel time or least effort. When adding a road to a complex network of roads, such as those that exist in a city, determining how a particular proposed route would affect travel distances and travel times is not always simple, but an understanding of such effects could be valuable in transportation planning. Fortunately, Geographic Information Systems software provides a tool for doing such analysis.
Although analyses of the Northfield road network have been done previously, no one has presented the results in terms of travel distances. Such an analysis can provide insight about whether the addition of roads is likely to encourage traffic patterns that are desirable to residents, businesses, and industry. This type of analysis can also be important from an environmental standpoint: The addition of any new road will have direct and indirect impacts on the environment, such as alteration of natural habitat or the initiation of new development. On the other hand, the addition of new roads may potentially decrease the on-road travel distance between locations, with possible environmental benefits in the sense that the minimum travel distance between locations is related to practicality of foot and bicycle traffic, as well as being an efficiency issue related to the amount of gas used by motor vehicles. The GIS analysis technique provides a way to quantify the effects of new roads and bridges on travel distance, which can be helpful in assessing costs and benefits--environmental and otherwise--involved in expanding the transportation network.
For these reasons, I used ArcView software in conjunction with data from the Northfield GIS available at Carleton College to create a series of maps which quantify how the proposed roads of the Comprehensive Plan would affect on-road travel distances from any part of the road network to a series of destination locations.
Geographic Information Systems are essentially collections of digital maps. With GIS software such as ESRI's ArcView, these maps can be stacked, viewed, and used for various kinds of spatial analysis. The analysis function to determine travel distance is performed on a map composed of a grid of cells. For this study, each grid cell represented either an on- or off-road location. Each cell has a value associated with it which represents the cost to move across that cell. For this study, the road cells had a value that corresponded to the size of the cell in meters. (It is also be possible to make use of information such as speed limits in order to assign a cost to each cell that represents the travel time across the cell.) After defining a point that acts as a destination, ArcView considers every possible route to determine the least costly (in this case, shortest) route from each cell in the map to the destination location. ArcView then creates a new grid in which each cell has a value that equals the distance to the destination along the shortest route.
Here's a summary of the procedure used to determine travel distances, complete with links to maps which show the procedure graphically:
The following maps show the results of the GIS analysis. The
grid cells on each of the following maps represent the decrease in
travel distance (between the cell and the destination) that would be
brought about by the addition of the proposed roads.
Effect of proposed roads on travel distance to the following
destinations:
For the downtown destination, the decrease in travel distance brought about by the proposed roads would be negligible. The maximum change in distance to destinations near the edge of town is less than one mile, a distance that can be covered in less than 2.5 minutes at 25 miles per hour. The largest change appears on the map with Highway 19 west as the destination, and the effects are clearly a result of the proposed Cannon River bridge/Jefferson Parkway extension.
The results of this study suggest that the travel distance benefits obtained by the addition of the proposed roads would be relatively minor. The greatest reduction in travel distance would occur as a result of the proposed Cannon River bridge. However, this alignment would also come at the greatest cost, environmentally and economically. (See the environmental considerations section of this web site.) Careful consideration should be made regarding whether the relatively short reduction in travel times are worth the environmental, economic, and community costs.
The goal of reducing travel distance is certainly not the only reason for the construction of new roadways, and this study suggests that travel distance reductions are likely not a primary benefit. Therefore, proposed transportation should also be assessed in terms of the other foreseen benefits. These may include:
Relief in the congestion of traffic.
- Would a bridge really help to accomplish this goal?
- Would it encourage more traffic, rather than simply spreading out the existing traffic?
- Are there alternative ways to accomplish this goal, such as encouraging pedestrian, bicycle, and public transport?
Routing of truck traffic around downtown Northfield.
- Is this a desirable goal, given that non-truck traffic would also be diverted from Northfield's downtown businesses?
The results above demonstrate that GIS analysis of the road network could be useful in Northfield transportation planning. Potential exists to expand the study in many directions:
Experimentation with other road configurations. Are there alternative road configurations that would better meet the needs of Northfield? Alternative routes be designated and tested to provide comparisons.
Analysis of more destinations. I focused on the need to get to locations at the edge of town, and used only one destination within Northfield. Other locations would be interesting to consider. These could include the hospital site, parks, and schools.
Travel time based on speed limits of roads. Speed limits could be used as an approximation of travel speed on roads to better determine how the proposed routes would affect travel time. (To do this analysis, each grid cell has a value that represents the travel time across the grid cell, rather than the size of the grid cell.)
Travel distance and travel time for pedestrians and bicyclists. Because there are several trails limited to foot and bike traffic that connect loop roads and cul-de-sacs, a study focusing on pedestrian and bike traffic could be used to identify strengths and weaknesses of the road/trail system. A map of existing trails and a digital version of the Trail Master Plan would be useful in carrying out this study. Furthermore, average values for walking and biking speeds could be used to make maps of travel time as well as travel distance.
Analysis in combination with traffic flow data. The method I used provides a means by which to study the direct effects of the proposed routes on travel distance, but the indirect consequences of the changes in time due to effects on traffic flow and changes in congestion are not as immediately obvious. Integrated analysis incorporating present traffic flow information, projections of future traffic volumes, and the GIS travel distance and travel time analysis could provide a more complete picture of the impacts of new roads on travel.
Comparison of street layout styles. Areas of grid vs. areas of winding streets and cul-de-sacs are obviously different in the extent to which they allow flow of traffic through an area. Distance calculations could be done on local areas of grid and cul-de-sac streets to quantify this difference.
Joanna Reuter, ENTS Independent Study, Spring 2000