Drive-Time Zones

The Drawing - Drive-Time Zones menu selection is enabled when the optional Business Tools extension has been installed and the focus is on a drawing window or on a drawing layer in a map window that contains lines and at least one point (which can be used as a center). If you have not activated the Business Tools extension with a valid Business Tools serial number you will not be able to use the Drive-Time Zones command.

Manifold's Drive-Time Zones command can find zones from one or more points, called Centers from which travel occurs. A drive-time zone is the distance that can be traveled through the road network (and possibly off road as well) in the given time.

The drawing of road lines used must be a true road network that is a connected network. If there are small breaks between the road lines or if the ends of adjacent lines are not exactly coincident the drawing cannot be used as a road network.

Controls

Center	Choose a set of points (usually the [Selection] or a saved selection) that are to be used as centers for the drive time zones. Very Important: Points to be used as centers must be exactly coincident with the end of one of the lines in the road network.
Roads	Choose a set of lines to be used as a road network. Usually [All Objects] to use all lines in the drawing, or might be a saved selection of lines to use only some lines from a very large drawing if we know that the drive time zones will end up being reasonably local.
Length	Choose a field to use that gives the length of each line in the road network. Choose the unit of measure used for the value in the field. Use the intrinsic field Length (I) if there is no explicit length field. On large road networks for which drive-time zones may be repeatedly calculated, it is more efficient to create a "length" field in the table and to copy into that length column the contents of the Length (I) intrinsic field using the transform toolbar for tables. This will avoid a recalculation of Length (I) on the fly for each run and will improve performance.
Speed	Choose a field to use that gives the speed of travel for each line in the road network. Choose the units of measure used for the length and time values in the field, for example, miles per hour or kilometers per hour. By default, if a column named Speed is available in the drawing's table it will be automatically loaded into the Speed box.
Split roads at intersections	If checked, split road lines into separate lines at intersections with other roads.
Method	Choose Buffer, Hull (default) or Zone as follows: Buffer - Find all portions of roads that may be reached in the given time and then build a buffer zone area about each road line that extends out the given distance. Specify the distance desired and the units to be used (meters, feet, etc.) for the distance. Use with care, since phenomenally complex buffer zones can be created in urban areas that may take a long time to compute. Hull - Find the most distant spot on each road that may be reached in the given time and create an area that is the convex hull enclosing the most distant points. This is the usual "drive time zone" polygon created by ordinary GIS systems. Zone - Find the most distant spot on each road that may be reached in the given time as well as the additional area that may be reached by travelling from each reachable spot at the given Off-road speed. The speed is specified using whatever units of measure are used for the Speed combo. The Zone method serves the classic task of trying to find how far someone can get in a given time if they travel either/or by road or by leaving the road and travelling over the ground off-road (often used for law enforcement search planning).
Zones	The drive time for each zone and the units of measure (hours, minutes, etc.). For example, to create a single drive time zone enter 60 for the distance reachable in that time. Enter 60, 120, 180 to create three drive time zones each of which is reachable in times of 60, 120 and 180.

To create a drive time zone from a single center:

1. Open a projected drawing that contains lines that form a connected road network and at least one point. Lines should have a field that can be used as the Speed field. If all roads are to be treated as having the same speed, simply create a "speed" field and fill it with 1 for all lines.

2. Select a point on the road network (lying exactly at the end of one of the lines that make up the network) that will be the center of the drive-time zone. Launch the Drive-Time Zones command.

3. Choose [Selection] for the Center value.

4. Choose [All Objects] for the Roads value.

5. Choose Length (I) to use as the Length value as well as the unit of measure.

6. Choose the name of the field giving the speed value in the Speed combo as well as the units of measure.

7. Choose the desired Method.

8. In the Zones field, choose the time desired for each zone plus the unit of measure. multiplied by the meter to kilometer or feet to miles value for the units of measure in use. Press OK.

When choosing Method, it is a good idea to start with a simple method, such as Hull, to see just how much of a region will be covered by a given set of parameters in a drive-time zone. This will reveal situations where settings accidentally result in very large drive-time zones that may cover many streets in an urban area. In such cases, use of the Buffer method may create extremely complex buffer zone areas that can take a long time (many hours) to compute. It is a good idea to test the desired parameters using a rapidly computed method such as Hull before launching a more compute-intensive method such as Buffer.

Example

We begin with a drawing of streets that has several points, one of which has been selected. The drawing is in an Orthographic meter-based projection so that distances are in meters and may be measured accurately.

$images\eg_drive_time_01.gif$

The streets (as usual) consist of many lines with the end points of the lines coincident to form a connected network, that is a real network without any gaps or overshoots between the lines. The points are located exactly at the ends of lines so that they two are part of the network.

$images\eg_drive_time_01a.gif$

If we open the drawing's table we see that most lines have a speed of 10 marked on them with some lines having higher speeds. In the drawing, lines with faster speeds have been thematically formatted so they are in different colors. The drawing's table also has the Length (I) intrinsic field showing as well as a Length field that we created and populated by copying the contents of the Length (I) intrinsic field. We could use the Length field if we had a really big network and wanted the calculations to go slightly faster, but it is not used in this example.

$images\eg_drive_time_02a.gif$

Launching the Drive-Time Zones command with the above settings will create a drive-time polygon in the shape of a convex hull about the selected point that was used as the Center.

Note that the units used are mixed, with meters, miles per hour and minutes in use. The Length parameter is in meters, since the use of Orthographic projection cast the native dimensions in use in this drawing in meters. The units used for Speed are simply whatever is intended by the values in that column in the drawing. The example drawing is of a location in the United States, where speeds are recorded in miles per hour.

$images\eg_drive_time_02.gif$

Manifold simply reaches out from the center point as far as is possible to go through the road network and then draws a polygon to enclose the most distant spots reachable. Because the two colored roads have much faster speeds than the smaller streets around them, by far the greatest reachable distance is over the faster roads. The maximum distance reachable in the given time over the faster roads thus marks the corners of the resultant drive-time zone.

$images\eg_drive_time_03a.gif$

If we delete the convex hull drive-time zone area and re-run the command using the same central point but using the Buffer option, we can create a drive-time buffer using a setting of 30 meters.

$images\eg_drive_time_03.gif$

The result is more interesting than using the Hull option. What we see is how far we can get from the center point along all of the roads in the given time, plus a buffer zone that extends out 30 meters from each reachable spot. Note that because of the great difference in speed between the main roads and the smaller streets, we can get only a small distance down any of the side streets in the given time.

The Buffer option is often used to find which addresses can be reached in a given time from a central location. Addresses are often placed in drawings using the geocoder and so they are often not exactly on the street line but close to the street line. By creating a buffer zone as a drive-time zone, we can find which address dots are located within the resultant buffer zone area.

Using the Buffer option provides a very rich display, but it can be very computationally intensive. In a road system with many roads, even a slight increase in the drive time distance can result in a considerably more complex buffer area with a surprisingly large increase in computation time required. Use this option with great care, scaling up in small steps to avoid launching Manifold on a job that could take days to compute.

$images\eg_drive_time_04a.gif$

A third option is to use the Zone option as seen in the dialog above, using an Off-road speed of 2. The units for the Zone speed are taken to be the same as are specified for the Speed parameter, in this case being miles per hour.

$images\eg_drive_time_04.gif$

The resultant drive-time zone is very interesting. It shows how far one can travel from the central point using a combination of travel on the roads and travel off the road. For regions far from the central point the zone shows those spots reachable when travelling all the way or almost all the way by car at the given road speed. That's why the zone is elongated along the lengths of fast main roads.

Closer in to the central point the zone covers those spots that are reachable if we go a short way by the road network and then get out of the car and travel off-road over the ground at the given off-road speed for whatever time is left. Therefore, close to the central point the regions between roads get filled in because they can be reached within the given time by off-road travel.

$images\eg_drive_time_05.gif$

If we had re-run the command using an off-road speed of 5 we would have obtained the result above. It shows that if our off-road speed is faster, we can get to even more regions in between roads.

The Zone option is often used by law enforcement or search and rescue services to define an area to search when a period of time has elapsed and the target person may be travelling on or off roads. For example, suppose some people in a four-wheel drive vehicle are lost in a semi-desert region where it is not clear if they stayed on the road during their travel or ventured off-road into the desert. If we know where they started and how much time has elapsed we can make reasonable guesses at their speed on the road network and their speed off-road. We can then use the Zone option to find the region to search, consisting of that region that they could reach through any combination of on-road and off-road travel.

Troubleshooting

Simple errors that may cause problems:

· Points to be used as locations are either not on lines or are not within the drawing's precision factor distance of a line. Since the default precision factor for a projected drawing is 0.000001 meter (about one thousandth of a millimeter) it is highly unlikely that points placed in drawings using anything other than Snap functions will be adequately close to a line to be considered "on" the line.

· The drawing of roads might not be a real network, that is, the ends of adjacent lines may not be coincident, there may be dangles or undershoots, breaks in the lines or other disconnects that prevent the lines shown from being used as a real network. Use Normalize Topology to "clean" the road network if need be, but keep in mind that poorly drawn road drawings might not be repairable even with the Normalize Topology tool.

· The units of measure used in the dialog do not correspond with the units of measure used in the drawing or used in the Speed or Length fields. For example, if a drawing has been projected using a foot-based projection the use of meters to interpret a Length (I) intrinsic field will be inaccurate.

· The drawing is not projected and covers a sufficiently large area that errors arise. Always use a drawing that has been projected using a projection appropriate to the region of interest. For small regions (say, the size of a state or province in most countries), Orthographic is a fine meter-based projection that is a good bet.

Note

Strictly speaking, a drawing used for drive-time zone computation need not be projected. However, since degrees of Latitude vary in size from the Equator to the Poles any task that requires accuracy should use only projected drawings.