Parallel route usage & improvements

Parallel route usage and improvements manage traffic shifting from congested roads to underutilized roads, in order to reduce congestion and create route efficiencies.

Key characteristics






WSDOT regions


Strategy description

Primary roads that run parallel to highways often have underutilized vehicle capacity. This is most evident during incidents or abnormal traffic conditions on popular routes. Shifting drivers to other routes can help to improve travel times, reduce congestion, and keep traffic moving efficiently.

Parallel routes can be made more usable by increasing their capacity or integrating their operations with the primary route through coordinated signal timing.

Parallel route improvements may also include adding traveler information systems that provide real-time status updates to travelers during trip planning and en-route. Broadcast media, website, or social media-based traveler information may encourage drivers to defer their trip completely, alter their route, or opt to travel via a different mode before they set off. En-route traveler information, provided by dynamic roadside signs or in-vehicle navigation applications, can direct drivers to alternate parallel routes.

When to use this strategy

These parallel route usage strategies are beneficial for congested corridors where the parallel routes have capacity, but are not commonly utilized (especially during incidents or abnormal traffic conditions). However, pushing traffic onto smaller streets that are not intended for additional vehicles, such as residential streets, can result in disruption to local traffic and upset members of the community.

Strategy benefits:

  • Increases capacity or access by diverting traffic to underutilized roads, resulting in more efficient use of existing capacity
  • Supports emergencies & unplanned events by opening the availability of redundant and detour routes in case of a closure or incident on one route

What you need in order to implement

Agency coordination needs:

  • The means to coordinate across jurisdictions on appropriate strategies
  • Agreement among the jurisdictions and agencies operating within a designated area about when to use the strategy, as well as what the specific approaches and response plans will be

Operations and technology needs:

  • Operational knowledge of the status of the road network, typically through traffic detection and a regional traffic management center (TMC) that oversees the system
  • Sign infrastructure to ensure that reliable alternate route information can be provided to drivers
  • Specific signage or physical street design changes for those streets that should not be part of rerouting/diversion strategies
  • Communications infrastructure and data standards that enable coordinated operations across multiple systems, agencies, and jurisdictions

Learn more about this strategy

U.S. Department of Transportation, The Role of Transportation Systems Management & Operations in Supporting Livability and Sustainability Chapter 4, Putting It All Together.

U.S. Department of Transportation, Integrated Corridor Management, Transit, and Mobility on Demand.

About key characteristics

Location notes:

Improving usage of parallel routes works well on busy highway and major road networks with viable alternate routes. It can be implemented quickly in regions with traffic management centers, and areas with robust centralized signal systems and communications infrastructure. 

Cost notes:

There are a wide range of costs depending on the extent of signage, detection equipment, and systems integration work required.

Technology notes:

Parallel route usage systems require reliable traffic detection, robust communications networks to share data between various agency systems, and sophisticated traffic management systems to generate alternate route suggestions during a traffic incident.

Collaboration notes:

Because facilities and services along a primary road are often independently operated by different jurisdictions and modal agencies, the concept of redistributing trips off of a primary road to a parallel route requires collaboration. Building intra-agency relationships and creating formal memoranda of understanding (MOU) are great ways to achieve this collaboration. Collaboration between agencies can then be used to further technical integration through consistent operating procedures, communications networks and data sharing, signal timing coordination, and planned responses for incident management.