Alternate Route Handbook

4. Alternate Route Selection (continued)

IDENTIFY PRELIMINARY ALTERNATE ROUTES

After stakeholders assemble and index data, they may then use the data to identify a set of preliminary alternate routes. Because more detailed analysis can have a significant cost in terms of both time and money, it is important that care be taken in the selection of preliminary alternate routes and that only feasible routes are selected. Table 4-4 shows possible roles of potential stakeholders for this step.

Table 4-4. Stakeholder involvement in identifying preliminary alternate routes

STAKEHOLDER	ROLE
Transportation/public works agency	As the lead agency, schedule meetings where stakeholders involved can meet and determine which alternate routes to identify for further consideration.
Law enforcement	Advise the lead agency to eliminate any routes from consideration that may involve safety issues or that would drastically affect response time.
Fire department	Advise the lead agency to eliminate any routes from consideration that would drastically affect response time.
Emergency medical service	Advise the lead agency to eliminate any routes from consideration that would drastically affect response time.
Transit agency	Help the lead agency consider alternate routes that would be beneficial to existing transit service routes. Identify separate preliminary alternate routes for use by transit operators if the primary alternate route is not suitable for transit route operations.
Turnpike/toll authority	Serve as lead agency if the alternate route is for a toll road. Determine whether the toll road is feasible as an alternate route. Determine the feasibility of waiving tolls, which can influence whether or not the toll road may be used as an alternate route.
Planning organization	May serve as the lead agency. Provide prior performed studies that may be useful in determining the feasibility of an alternate route. Provide knowledge about an entire area, rather than just a single jurisdiction.

Minimum Actions for Identifying Preliminary Alternate Routes

The task of identifying preliminary alternate routes involves the following minimum action: obtain stakeholder consensus on at least one preliminary alternate route to be studied further. Stakeholders should choose one or more preliminary alternate routes that warrant further study. The alternate route must meet all criteria set in the previous step. Selecting multiple routes in this step will allow additional routes to be investigated in the next step if the site visit eliminates the primary route as a
viable candidate.

Ideal Actions for Identifying Preliminary Alternate Routes

The following ideal actions may be applied in addition to the minimum action:

• Use travel demand models to estimate the traffic volume that will use the potential alternate routes. Associated considerations include:
  • For major route disruptions, travel demand models will create new trip tables that reflect changes in the traffic flow. By removing a key highway link, the demand model will change the network times and generate new traffic flow patterns. The revised network flow pattern should then be used as input to a traffic simulation model in order to better estimate how the highway closure affects operations after accounting for changes in travel demand.
    
  • Shortest (time or distance) alternative routes can be identified using the traffic assignment module in demand models. In the demand model's traffic assignment process, the mathematical algorithms are based on finding the shortest paths between two points. Given sufficient data, GIS software can accomplish this step as well.
    
  • For a typical application in identifying preliminary alternate routes, the roadway segment that will be closed or blocked is eliminated from the model network, and the assignment module is run. The modeler then identifies the alternate routes that best meet the selection criteria, including the routes with the shortest travel distance, the shortest time, or the highest capacity.
    
  • Travel demand models can also identify alternate routes that will have the minimum impact on emergency response services. The location of emergency services is identified on the network, and travel time runs using the assignment module could determine the potential change in response times to the community service area.
    
• Obtain stakeholder consensus on multiple preliminary alternate routes to be studied further. Stakeholders should choose multiple preliminary alternate routes that warrant further study, and must meet all criteria set in the previous step. Selecting multiple routes will allow additional routes to be investigated in the following step if the site visit eliminates the primary route as a viable candidate.

CONDUCT ALTERNATE ROUTE SITE VISIT

Although maps and databases may contain useful information, no map or database can capture how a route operates on a day-to-day basis. Detailed information on roadside development and traffic patterns is rarely available through a map or database. In addition, maps or databases may have errors or could be misleading: a street labeled as a major arterial on a map may be considered as an alternate route, but the street may actually be a narrow, unpaved road that is not feasible as an alternate route. A site visit would generally be the responsibility of the lead agency. However, the agency responsible for the roadway being considered should participate in the site visit.

After stakeholders conduct the site visit, they are in a much better position to determine the feasibility of the alternate routes that have been proposed. If none of the routes that were chosen meet the alternate route selection criteria, then stakeholders must return to the Assemble and Index Data step in order to identify another set of preliminary alternate routes. Table 4-5 shows possible stakeholder roles for conducting the site visit.

Table 4-5. Stakeholder involvement in conducting alternate route site visit

STAKEHOLDER	ROLE
Transportation/public works agency	As the lead agency, send personnel into the field to conduct the site visit.
Law enforcement	Obtain safety-related information through the site visit.
Fire department	Provide input on response times depending on which alternate route is selected.
Emergency medical service	Provide input on response times depending on which alternate route is selected.
Turnpike/toll authority	Participate in a site visit of a toll road designated as a candidate alternate route.
Freeway service patrol	Patrol the freeway system. Obtain information on locations with safety problems as well as information on the general operation of the freeway.

Minimum Actions for Conducting an Alternate Route Site Visit

The task of conducting an alternate route site visit involves the following minimum action: conduct a site visit to observe how the candidate alternate routes perform in the "real world" and to determine if these routes meet the selection criteria. Associated considerations include:

• Stakeholders should visit and drive each of the alternate routes to identify features that do not appear on a map or in any database. It is especially important to ensure that all roads comprising the route are physically connected and that traffic flow is possible in the direction required by the alternate route. For example, two roads may appear to cross each other on a map, although there may be an impassable median that blocks the turn needed for the alternate route.
  
• The stakeholder performing the site visit should ensure that the geometry of the alternate route is able to accommodate all vehicle types. Features to look for include low overpasses, narrow bridges, bridges with weight limits, and tight ramps that may be restrictive to commercial vehicles.
  
• The stakeholder performing the site visit should check that the alternate route has adequate capacity to accommodate diverted traffic volume. The stakeholder should locate potential problem areas, such as an unsignalized left turn onto a heavily traveled street or against heavy oncoming traffic. The stakeholder must determine if these chokepoints can accommodate diverted and background traffic demand or if additional transportation management planning is required at these locations to affect a temporary capacity increase during alternate route implementation.
  
• Railroad grade crossings should be located. Trains that traverse the grade crossing on a regular basis could make the alternate route unviable.
  
• Major traffic generators (such as a large shopping center) should be located. Heavy traffic volumes near these major traffic generators could significantly reduce the level of service at varied times. The type and intensity of roadside development represents another important factor. Alternate routes with schools and hospitals should be avoided or only considered during specific periods.
  
• Stakeholders should schedule site visits during morning and evening peak periods, as well as during midday and nighttime hours, to gain an accurate understanding of how the alternate route may function during different times of day.
  
• Other stakeholders may assist with the site visit as part of their daily routine. Freeway service patrols, law enforcement, and transit operators may drive the portions of candidate alternate routes each day and may be able to take special notice of conditions and features that affect the feasibility of implementing a candidate alternate route. Transit operators may provide input as to whether or not an alternate route is feasible from the perspective of potential impact on transit operations. Public safety agencies may also visit the site to determine whether the use of any potential alternate route will negatively impact response times.
  
• Freeway Alternate Routes Only: Typical bottlenecks on a freeway alternate route include ramp connectors and weaving areas. The stakeholder performing the site visit should check the geometry and operation of ramp connectors and weaving areas to ensure that these points have adequate capacity to accommodate the traffic.

Figure 4-3 shows an example of an analysis that may be performed after the site visit to determine whether or not any acceptable alternate route exists.

Figure 4-3. Alternate route analysis and recommendations. (Source: North-Central Texas Council of Governments)

Ideal Actions for Conducting an Alternate Route Site Visit

In addition to conducting a site visit, consider the following ideal action: videotape site visits in order to review the visits at a later time. Ideally, the stakeholders performing the site visit should videotape the site visit so that other stakeholders may review the site. The videotape proves useful for presentation at future stakeholder planning meetings to select alternate routes, and should show conditions during both peak and off-peak periods.

EVALUATE PRELIMINARY ALTERNATE ROUTES

After stakeholders conduct the site visit, they must evaluate the performance of each remaining candidate alternate route. The level of analysis that is possible depends on the availability of funding and personnel to conduct this step. The more traffic operations data available on the route, the easier it is to apply a suitable analysis technique to evaluate the operation of the route under implementation conditions.

After stakeholders evaluate the candidate alternate routes, they must decide whether an acceptable alternate route is available based on previously established criteria. If none of the alternate routes studied are acceptable, then stakeholders must return to the Assemble and Index Data step in order to identify a new set of possible alternate routes. Because of the level of effort and funding required in subsequent planning steps, stakeholders should exercise care in evaluating preliminary alternate routes to avoid having to repeat this process more than once.

Table 4-6 describes possible roles for stakeholders involved in this step.

Table 4-6. Stakeholder involvement in evaluating preliminary alternate routes

STAKEHOLDER	ROLE
Transportation/public works agency	As lead agency, perform capacity analysis and/or computer traffic simulation to evaluate the traffic conditions on alternate routes during diversion. Provide an estimate of traffic diverted by time of day and day of week.
Law enforcement	Evaluate the safety of alternate routes when used. Evaluate both traffic safety and personal safety of motorists.
Fire department	Evaluate their response time when the alternate route plan is in effect. Consider both their response time to the incident, as well as response time to other emergencies that may occur at the same time in different locations.
Emergency medical service	Evaluate their response time when the alternate route plan is in effect. Consider both their response time to the incident, as well as response time to other emergencies that may occur at the same time in different locations.
Transit agency	Evaluate transit performance along the alternate route. Provide information on transit schedules to the lead agency so that transit can be included in alternate route evaluation activities.
Turnpike/toll authority	Provide traffic data on the toll road to the lead agency. Evaluate how much revenue will be lost if tolls are waived. Determine the feasibility of waiving tolls.
Elected officials	Help the lead agency choose an alternate route that, when implemented, creates a negligible impact on community quality of life.
Planning organization	Examine prior performed studies, which may be useful for evaluating alternate routes. Use travel demand models to evaluate alternate routes.
Individuals and community groups	Advocate beneficial alternate routes, both from the point of view of motorists and as members of the community. Provide firsthand knowledge of the alternate routes that is useful for the lead agency.
Freeway service patrol	Patrol the freeway system. Provide safety data on the freeway under consideration as an alternate route. Provide general information on the operating characteristics of the freeway.

Minimum Actions for Evaluating Preliminary Alternate Routes

The task of evaluating preliminary alternate routes involves the following minimum action: conduct a capacity analysis of candidate alternate routes. Associated considerations include:

• Stakeholders should conduct capacity analyses for different times of the day using applications based on Highway Capacity Manual (HCM) methodologies.¹
  
• For a freeway, locations to analyze include ramp junctions, weaving segments, and general freeway sections in order to identify the location with minimum capacity (bottleneck location capacity).
  
• For a street, both the street as a whole should be analyzed (using the analysis method for arterials) as well as individual intersections.
  
• At a minimum, these analyses will require traffic volume data and roadway configuration data.

Ideal Actions for Evaluating Preliminary Alternate Routes

In addition to conducting capacity analyses, consider the following ideal action: evaluate candidate alternate routes using traffic simulation software, and test a variety of scenarios. Associated considerations include:

• Traffic simulation is a useful tool for studying how candidate alternate routes can accommodate traffic diverted from the primary route. Because field studies are usually not feasible for this purpose, traffic simulation represents the best available tool. Network or corridor simulation can be used to identify bottleneck locations, and HCM-based or other software can be used to further study bottleneck operations.
  
• Stakeholders should conduct route capacity analysis for different times of the day and days of the week using traffic simulation models.
  
• Stakeholders should model both mainline and alternate routes at the same time as a network. The bottleneck location may likely reside at junction points between the alternate route and the primary route.

The FHWA Traffic Analysis Tools Primer² provides guidance in the selection and application of the following types of computer traffic simulation models to various scenarios, including (1) macroscopic simulation models, (2) mesoscopic simulation models, and (3) microscopic simulation models.

Macroscopic simulation models are based on deterministic relationships of flow, speed, and density of the traffic stream. Macroscopic models simulate traffic on a section-by-section basis, rather than tracking individual vehicles. Macroscopic models produce less detailed results than mesoscopic or microscopic models but require the least amount of storage space, computing requirements, and simulation time. Examples include TRAF-CORFLO and FREQ.

Mesoscopic simulation models combine properties of both macroscopic simulation models and microscopic simulation models. Mesoscopic models simulate individual vehicles, but vehicle speed is determined by the average speed of the link.

Microscopic simulation models simulate movement of individual vehicles, based on theories of car-following and lane-changing. Microscopic simulation models require more storage space, computing requirements, and simulation time than macroscopic or mesoscopic models but produce the most detailed results. Examples include CORSIM and VISSIM.

SELECT PREFERRED ALTERNATE ROUTE

After stakeholders evaluate the candidate alternate routes, they can select the preferred alternate route from among remaining feasible routes. If possible, stakeholders should choose a secondary and tertiary alternate route in addition to the primary alternate route. In cases where the primary alternate route is unavailable or impeded, the secondary and tertiary alternate routes could be used to augment use of the primary alternate route. All stakeholders should be informed of the decision, even if their approval is not required.

Table 4-7 describes possible roles for stakeholders involved in this step.

Table 4-7. Stakeholder involvement in selecting preferred alternate route

STAKEHOLDER	ROLE
Transportation/public works agency	Set up stakeholder meetings where stakeholders may select the preferred alternate routes. Assure that all transportation agencies involved sign off on the selected alternate routes and subsequent alternate route plans.
Law enforcement Fire department Emergency medical service Emergency management agency/Homeland Security agency Transit agency Turnpike/toll authority Elected officials Planning organization Individuals and community groups Major incident response team	Participate at meetings where stakeholders select preferred alternate routes. Sign off on the selected routes and subsequent alternate route plans.

Minimum Actions for Selecting a Preferred Alternate Route

Minimum actions for selecting a preferred alternate route include:

• Obtain stakeholder consensus on the selection of the best available alternate route. After stakeholders evaluate the potential alternate routes, they should select the best available alternate route that meets established selection criteria and provides satisfactory level of service under implementation conditions.
  
• Notify (by lead agency) all affected stakeholders about the alternate route selected. Even if a stakeholder's concurrence is not required for the alternate route selection process, the lead agency should notify that stakeholder of the alternate route that has been chosen. All of the stakeholders listed in table 4-7 should be notified, particularly the following groups that may not have participated in the route selection process:
  • Agencies from adjacent jurisdictions that may experience impacts from diverted traffic.
    
  • Major businesses along the alternate route.
    
  • Emergency services from adjacent jurisdictions that may use the alternate route.
    
  • Media.

Ideal Actions for Selecting a Preferred Alternate Route

The following ideal actions may be applied in addition to the minimum actions:

Figure 4-4. Alternate route plan showing a primary and secondary alternate route. (Source: Florida DOT)

• Choose at least two (preferably three) alternate routes, and obtain stakeholders' concurrence on the selection. Associated considerations include:
  • Ideally, stakeholders should choose at least two alternate routes: a primary and secondary alternate route as shown in figure 4-4.
    
  • If possible, stakeholders may choose three alternate routes. This is useful in the event that the volume of diverted traffic creates an unacceptable level of service on the primary alternate route, and additional routes are needed during implementation to alleviate demand on the primary alternate route.
    
  • Secondary and tertiary alternate routes also prove valuable if a concurrent event, such as an incident, construction, or flooding, affects available capacity on the primary alternate route.
• Review alternate routes to decide if they are still effective or if a new alternate route should be selected. Associated considerations include:
  • Stakeholders should consider an alternate route plan as a living document, given that the best alternate route may change over time as a result of roadway infrastructure improvements or deteriorating day-to-day operation of the designated alternate route.
    
  • Stakeholders should meet regularly to review alternate routes and make revisions, if necessary. A few situations that may require selection of a new route include:
    • Community opposition to the current alternate route.
      
    • Increased traffic volume on the alternate route.
      
    • Construction activities that temporarily reduce capacity on the alternate route.
      
    • Construction of a new road that could represent a better alternative to the selected alternate route.
      
    • New development along the alternate route.
      
    • Security issues.
      

References

1. Highway Capacity Manual, 2000 edition, Transportation Research Board, Washington, DC, 2000.
   
2. Traffic Analysis Tools Primer, Federal Highway Administration, Washington, DC, January 2003.