Alternate Route Handbook

4. Alternate Route Selection

Figure 4-1. Using a demand model in alternate route selection.

INTRODUCTION

The first phase of alternate route planning is alternate route selection. Chapter 3 identified the following seven steps that must be completed in order to select alternate routes that facilitate improved corridor operations without creating undue impacts on the community:

• Determine objectives
• Establish alternate route criteria
• Assemble and index data
• Identify preliminary alternate routes
• Conduct preliminary alternate route site visit
• Evaluate preliminary alternate routes
• Select preferred alternate route

DETERMINE OBJECTIVES

The first step in the alternate route selection process is determining performance and community-based objectives to guide planning activities and the alternate route selection process. Stakeholders must agree on what objectives they want to meet in developing the alternate route plans. Considerations include:

• Local or regional
  • Should the alternate route be geared toward local travelers who likely travel short distances along the main route and want to remain as close to the main route as possible?
    
  • Should it be geared toward long-distance, regional travelers who desire an alternate route that provides the lowest travel time to the next major populated area or major interchange (e.g., between two freeway facilities), regardless of its proximity to the main route or what cities the route passes through?
    
  • Should separate alternate routes be set up for local and regional travelers?
• Geographical area
  • What geographical area should the alternate route plan cover?
    
  • Should it encompass a single freeway facility or only a single freeway segment?
    
  • Should it consist of a series of plans for multiple facilities across a
    metropolitan area?
    
  • Should it be a Statewide plan?
    
  • Should it be coordinated with adjacent States, particularly if a metropolitan area crosses State lines?
    
• Alternate route facility types
  • Should freeways, arterial streets, and/or multi-jurisdiction roadways be considered as preliminary alternate routes?
• Interagency coordination
  • To what extent should the lead agency coordinate with other stakeholders?
    
  • Should the lead agency coordinate with a State transportation/public works agency to use State roads or a local transportation/public works agency to use city streets as an alternate route?
    
  • Should the lead agency coordinate with both State and local transportation agencies throughout the selection process?
    
• Frequency of alternate route implementation
  • Will it be used only in the case of a complete closure of the primary route for a prolonged period?
    
  • Will it be used whenever a lane closure occurs on the primary route during specific days and times?

In some cases, the objectives may change during the alternate route planning process. For example, the lead agency may initially intend to work alone, without much coordination with other agencies. However, while selecting an alternate route, the lead agency may determine that acceptable routes are under the jurisdiction of other agencies, thus requiring coordination with these agencies to effect optimal alternate route operation during diversion. Also, different stakeholders have different areas of expertise that may be useful in alternate route planning. For example, a transportation/public works agency may be the most knowledgeable about traffic conditions, while a law enforcement agency may be the most knowledgeable about safety problems.

Stakeholders involved in the selection process must coordinate their work efforts to facilitate a seamless and successful selection process. In most jurisdictions, the State DOT acts as the lead agency in the selection of the candidate and, after evaluation, the preferred alternate routes. In some jurisdictions, the State police may take the lead, or the State police and State DOT may co-lead.

ESTABLISH ALTERNATE ROUTE CRITERIA

After stakeholders identify the objectives, they may begin establishing alternate route criteria. Stakeholders must agree on what criteria alternate routes must meet before they may select the alternate route. Table 4-1 shows the stakeholders typically involved in establishing alternate route criteria and the roles they are likely to perform.

Table 4-1. Stakeholder involvement in establishing alternate route criteria

STAKEHOLDER	ROLE
Transportation/public works agency	Serve as the lead agency, since it is in charge of the road from which traffic is to be diverted. Set up meetings where criteria may be determined. Invite to meetings all transportation agencies in charge of roads that are being considered as alternate routes. Apply knowledge about traffic-related criteria. Establish criteria affecting the extent to which an alternate route may impact one of the roads under their jurisdiction. For example, some agencies may not want excessive traffic diverted onto their roadways.
Law enforcement	Determine safety-related criteria. Safety-related criteria may relate to both traffic safety as well as the personal safety of motorists and pedestrians. Set criteria affecting the extent to which their response time may be affected. For example, law enforcement may not allow the use of a roadway where police vehicles would be impeded from entering the roadway, since it may negatively impact response time.
Fire department	Set criteria affecting the extent to which their response time may be affected by the alternate route. Heavy traffic on an alternate route adjacent to a firehouse may impede fire trucks from entering the roadway, leading to an increase in response time to the scene, in addition to emergencies unrelated to the incident causing the use of the alternate route.
Emergency medical service	Set criteria affecting the extent to which their response time may be affected by the alternate route.
Transit agency	Set criteria affecting the extent to which transit operations (e.g., transit routes) may be affected by the alternate route. Advise the lead agency as to which routes have transit service and what the minimum criteria are for transit service to be able to operate along a route. For example, the transit agency may specify a maximum acceptable travel time.
Turnpike/toll authority	Serve as the lead agency if the alternate route plan is for traffic diverted to or from a toll road. Set up meetings where criteria may be determined. May have many of the same responsibilities and expertise that a transportation/public works agency may have, when a toll road is involved in an alternate route plan. Establish restrictions as to when tolls may be waived to accommodate alternate route traffic.
Elected officials	Advocate criteria that minimize community impact.
Planning organization	Apply knowledge about a region as a whole. Serve as the lead agency and coordinate meetings where stakeholders establish alternate route criteria. Set traffic-related criteria, as well as criteria affecting the extent to which roads within the region may be affected.
Individuals and community groups	Advocate criteria that benefit both motorists and the community.

Minimum Actions for Establishing Alternate Route Criteria

The task of establishing alternate route criteria involves the following minimum action: obtain stakeholder consensus on criteria for alternate route selection. Associated considerations include:

• An important minimum criterion is that the geometrics of the alternate route must be able to accommodate all vehicle types. Commercial vehicle restrictions and limited available turning radii that cannot accommodate certain vehicles must be identified. If these restrictions arise, the alternate route plan must make accommodations for vehicles that cannot use the alternate route.
  
• An alternate route must be reasonably close to the primary route in order to be useful. If the alternate route is too far from the primary route, motorists who are unfamiliar with the area may not be comfortable navigating the alternate route. In rural areas, it may be necessary for the alternate route to be farther away from the primary route, since close parallel roads may not be available.
  
• The alternate route must have sufficient capacity to accommodate the traffic that is diverted. For example, if traffic is diverted from a 6-lane urban freeway, a 2-lane local street may not have adequate capacity to serve as an alternate route.

Ideal Actions for Establishing Alternate Route Criteria

In addition to obtaining stakeholder consensus on criteria for alternate route selection, consider the following ideal action: set detailed criteria for alternate route selection in addition to the relative priority of each criterion. Associated considerations include:

• Criteria should be chosen to benefit both motorists and the community at large.
  
• Stakeholders must agree on the relative importance of each criterion. For example, some stakeholders may choose travel time as the most important criterion, while others may prefer a route with a greater travel time that causes less community disruption.

Table 4-2 summarizes various criteria for selecting an alternate route.

Table 4-2. Criteria for alternate route selection

CRITERION	ENTITY IMPACTED	ACTION
Proximity of alternate route to closed roadway	Motorist	Determine whether the alternate route is intended for local traffic or for regional traffic. For local traffic, it should ideally be in close proximity to the primary route. In a metropolitan area, the closest alternate route may be an adjacent parallel street or a freeway frontage road. In rural areas, alternate routes may be farther away from the primary route. Provide a time savings to motorists. If an alternate route is too far away from the primary route, then travel time may be longer than that on the primary route in some instances. For a regional alternate route, connecting successive cities or major interchanges served by the primary route, it is less important that the alternate route be in close proximity to the main route; however, the alternate route should not be significantly longer than the primary route.
Ease of access to/from alternate route	Motorist	Select access points between the primary and alternate route that do not create bottleneck points in the corridor. Consider alternate routes that provide high-capacity connections, or sufficient space and geometry to establish special traffic control during implementation, to/from the primary route.
Safety of motorists on alternate route	Motorist	Select routes that are easy for motorists to navigate and provide a sense of comfort. Long routes may be difficult for motorists to navigate Motorists may be uncomfortable using alternate routes that take them through unfamiliar areas and/or offer few service stations. Motorists may feel more comfortable driving on an alternate route where the primary roadway is visible, rather than driving through an unfamiliar area. Do not use a street that has known safety problems, unless it is patrolled by law enforcement to ensure the safety of motorists.
Height, weight, width, and turning restrictions on alternate route	Motorist	Consider roadways without physical constraints limiting the height, weight, and width of vehicles along the alternate route. Be aware that constraints may render an alternate route impassable for commercial vehicles. Choose an alternate route that is usable by all vehicles. If commercial vehicles cannot be accommodated on what is otherwise the best alternate route, then an additional route should be selected for commercial vehicles. Review operations of intersections that do not allow vehicles to make certain turns, especially left turns that may be required during alternate route operation. A turn that is normally banned may be allowed on the alternate route, using special law enforcement control and signage.
Number of travel lanes/capacity of alternate route	Motorist	Require sufficient capacity to accommodate the vehicles diverted while carrying day-to-day background traffic. For example, if traffic from a busy six-lane urban freeway is diverted to a two-lane local street, there may not be enough extra capacity on the street to accommodate the diverted traffic. Assure that diverted traffic does not encounter an even worse level of service than it would encounter on the primary route.
Congestion induced on alternate route	Motorist	Avoid routes where motorists do not realize a travel time savings because of demand-induced congestion on the alternate route.
Traffic conditions on alternate route	Motorist	Assure that an alternate route is not already operating near capacity, and does not have sufficient extra capacity to accommodate the diverted traffic. The diverted traffic should not encounter an even worse level of service than on the primary route.
Number of signalized intersections, stop signs, and unprotected left turns on alternate route	Motorist	Assure that signalized intersections, stop signs, and unprotected left turns do not cause substantial delay to motorists along an alternate route. An unprotected left turn or a left turn from a stop sign may also cause safety problems under heavy traffic conditions.
Travel time on alternate route	Motorist	Assure that the alternate route is free-flowing and is not excessively long relative to travel distance on the primary route, so that motorists can save time. Likewise, motorists may not save any time if travel time is long due to congested traffic, even if the alternate route has a shorter travel distance than that on the primary route.
Pavement conditions on alternate route	Motorist	Assure that good pavement conditions exist. Pavement conditions may be uncomfortable to motorists, cause safety problems, and even cause damage to vehicles. If pavement condition is already poor, then diverted truck traffic not normally serviced on an alternate route may further damage the pavement.
Type and intensity of residential development on alternate route	Community	Do not divert traffic to residential or mixed-residential streets, if possible. Residential streets are generally low capacity and are often not designed as through-streets. It is usually best to avoid the use of residential streets as alternate routes.
Existence of schools and hospitals on alternate route	Community	Consider impact on local driveway access. One side effect of alternate routes is that the increased traffic may increase the difficulty of local driveway access. For this reason, it is usually best to avoid the use of streets that serve schools and hospitals as alternate routes because it is important that easy access be maintained for these facilities. Consider impact of heavy traffic that may negatively affect ambulance access to hospitals. Consider the impacts of heavy traffic that may increase pedestrian/vehicular conflicts.
Percentage of heavy vehicles (e.g., trucks, buses, RVs) on route from which traffic is to be diverted	Motorist	Examine high volume of heavy vehicles that will significantly reduce available remaining capacity on the alternate route. The acceleration and operating characteristics of trucks may constrain traffic flow on the alternate route.
Grades on alternate route	Motorist	Examine impact of steep upgrades or downgrades that may cause safety problems, especially in bad weather. A steep upgrade can significantly reduce capacity on a roadway carrying a high volume of commercial vehicles because upgrades limit their speed.
Type and intensity of commercial development on alternate route	Community	Examine capacity constraints at heavy commercial developments, such as a shopping mall. If a large traffic generator is located adjacent to a candidate alternate route, then it may generate traffic demand that approaches or even exceeds available roadway capacity, thus making the roadway undesirable for use as an alternate route. Streets in commercial areas usually have a large number of unsignalized driveways, which cause both traffic and safety problems when volume is heavy.
Availability of fuel, rest stops, and food facilities along alternate route	Motorist	Consider that motorists may feel more comfortable using a route on which these facilities are available. On an extended or regional alternate route, motorists may wish to stop and eat, rest, and/or to refuel their vehicles.
Noise pollution	Community	Consider the impact of increased traffic that may significantly increase the amount of noise pollution along a route. A significant increase in noise level during alternate route implementation may cause unacceptable disturbance to affected areas of the community.
Transit bus accommodation	Motorist	Examine potential impacts on transit vehicle station stops due to increased volumes of diverted traffic.
Air quality	Community	Examine impact of increased traffic that may significantly increase pollution and decrease air quality. Remember, the goal of alternate route deployment is improving mobility and system operations.
Ability to control timing of traffic signals on alternate route	Motorist	Identify possible modification to day-to-day traffic signal timing plans in order to accommodate the additional diverted traffic. Choose an alternate route that allows an operator to modify remotely the timing of traffic signals upon alternate route deployment.
Ownership of road	Motorist/ Agency	Coordinate efforts among agencies responsible for operations on the primary route and the alternate route. If traffic is being diverted from a State road, it is desirable to divert traffic to another State road. The State can modify the traffic signal timing on a State road, whereas it may not be allowed to modify traffic signal timing on a county or local road. Diverting from one State road to another State road avoids jurisdictional difficulties.
Availability of ITS surveillance equipment on alternate route	Motorist	Consider the benefits of an alternate route having an ITS instrumented system. ITS surveillance equipment, such as CCTV cameras, allows an operator to monitor traffic conditions on an alternate route during plan implementation.
Availability of ITS information dissemination equipment on alternate route	Motorist	Utilize ITS information dissemination equipment, such as CMSs or HAR, to give motorists information on how to access the alternate route as well as traffic information required to navigate the alternate route and reach a downstream connection with the primary route.

ASSEMBLE AND INDEX DATA

After alternate route criteria are established, stakeholders may begin assembling and indexing data. If any of the agencies involved have access to a GIS database with pertinent information on roadway characteristics, it would be very useful for this step.

Stakeholders should review all the data available to ensure that the necessary data has been acquired to evaluate potential alternate routes on the basis of the selection criteria. Table 4-3 describes roles that different stakeholders may perform in the process of assembling and indexing data. It should be noted that roles may vary depending on the region and the ownership of roads that are involved in the alternate route.

Table 4-3. Stakeholder involvement in assembling and indexing data

STAKEHOLDER	ROLE
Transportation/public works agency	As lead agency, schedule meetings to determine what type of data to assemble and index. Use agency personnel to obtain the data. Provide maps and databases available with information on its roads. Utilize GIS database or demand model to help support this step.
Law enforcement	Provide available safety-related data to determine which sections of roadway are the highest priority for alternate route planning as well as to determine the safety of potential alternate routes.
Fire department	Provide locations of all firehouses and the routes they typically use for response. This will help avoid alternate routes that negatively impact response time.
Emergency medical service	Provide locations of all hospitals and dispatch facilities and the routes they typically use for response. This will help avoid alternate routes that negatively impact response time.
Transit agency	Provide information on bus routes and bus stations in addition to specific alternate routes that may be suitable or not suitable for transit.
Turnpike/toll authority	Serve as the lead agency if the alternate route plan diverts traffic from a toll road. If a toll road is being considered as an alternate route, provide maps, databases, and other available information.
Planning organization	May serve as lead agency. Coordinate meetings where stakeholders determine what type of data to index and assemble. Provide access to resources, such as a GIS database and demand models, which other agencies may not have in-house. Provide information about major roads throughout a region.
Freeway service patrol	Provide firsthand information on sections of freeway that have the most safety problems as well as overall traffic conditions on the freeways.

Minimum Actions for Assembling and Indexing Data

The task of assembling and indexing data involves the following minimum action: use a paper or electronic map to determine the location of roads that may be used as alternate routes and to obtain information on these routes. Associated considerations include:

• Stakeholders can use a commercially available paper or electronic map and locate all roadways that are near the route from which traffic is to be diverted. Stakeholders should also check access to and from these roadways to ensure that they connect to each other.
  
• Information that may be available includes roadway classification, pavement type, geometric information, traffic volumes, development density, and accident data.

Ideal Actions for Assembling and Indexing Data

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

Figure 4-2. GIS map for alternate route planning in Oklahoma. (Source: Geospatial Solutions)

• Use a GIS map, if available, to obtain detailed information on available routes. Associated considerations include:
  • GIS databases contain a variety of useful information on roads and other features. These features are shown graphically on an electronic map. Here, different features (layers) can be turned on or off as necessary, and symbols may be changed.
    
  • GIS maps, as shown for example in figure 4-2, are ideal for obtaining information on primary routes and candidate alternate routes.
    
  • GIS databases can save time in planning an alternate route by allowing the planners to obtain basic information about the candidate routes, which would otherwise be available only with a field study or through cumbersome research of paper maps and files.
    
  • Information that may be available through a GIS includes roadway classification, pavement type, geometric information, traffic volumes, development density, and accident data.
    
  • If a GIS database shows traffic incident locations, then it can be used to identify a high incident location for which an alternate route should be planned.
    
  • While the lead agency may not have access to GIS, it is possible that other agencies have access to GIS, which can be used in the alternate route planning process. Examples of such agencies include rural or metropolitan planning organizations, municipal or county planning departments, and utilities. If the lead agency does not have access to a GIS database, then they should ask other stakeholders if such databases exist and if they may use them to develop the alternate route plan.
    
  • Because no single database can include all information about every aspect of a roadway network, it is necessary to consult other sources, such as a traffic study, to obtain certain data.
• Consult planning organizations for access to travel demand models that can be used for obtaining data. Associated considerations include:
  • All travel demand models have a highway network; some larger planning organizations may also have demand models that include transit networks. The networks typically contain information for each link, such as number of lanes, capacity, length, speed limit, functional category (freeway, arterial), and area type (metropolitan, urban/rural). When available, many links will include traffic volume information. Transit links will include information such as route number, headway times, capacity, and fare structure.
    
  • Sometimes travel demand model networks may be even more readily available than GIS databases and may provide extensive roadway information. The demand model systems used by planning organizations have the capability of displaying the roadway networks in a layout similar to GIS. Some software transportation planning packages combine the capabilities of a GIS with travel demand models. These tools provide a quick and powerful way for the user to identify and display capacity, lanes, and speeds as part of the alternate route planning process for assembling and indexing data.
• Consult transportation agencies for information on typical traffic volumes, roadway geometry, signage, and traffic control devices. Transportation agencies may have detailed information on the roadways being investigated as potential alternate routes. This data will assist in applying the selection criteria to candidate alternate routes during the selection process.
  
• Consult law enforcement personnel and/or transit operators for information on traffic operations. Law enforcement personnel and transit operators may have firsthand knowledge of typical traffic conditions from policing and traveling the alternate route on a daily basis. Law enforcement may have records of travel speeds and incident data. Transit agencies may log travel times during different times of the day along portions of candidate alternate routes from scheduled monitoring activities. This data should complement data provided by the transportation/public works agency.
  
• Consult freeway service patrols operators for information on traffic operations and incident frequency. Freeway service patrols, namely its administering agency, may have records of incident occurrence, type, and duration. Also, since freeway service patrols travel a particular route on a daily basis, operators may have firsthand knowledge of typical traffic conditions at different times of the day and days of the week.

(Chapter 4 is continued on next page.)