Dual Vs. Single Carriageways

State departments of transportations (DOT’s) all must report on several items for the HPMS Report. The HPMS report details everything from total public road mileage, to pavement quality to traffic data. An important part of this report is the route network, or the ARNOLD network. ARNOLD is a newly required part of HPMS where every road in the state must be Geographically and Spatially referenced. This requirement has increased the GIS for all states and unifies the GIS to include local roads and some private roadways. With this requirement, states now must have a Linear Referencing System (LRS) that is able to detail and represent all roadways in an accurate and appropriate way, a large part of this is the representation of carriageways in the LRS. A carriageway is the route line that represents the roadway and how it is delineated in the LRS. This is a divided facility that moves traffic in opposite directions. This can be contentious as each state represents carriageways differently. This story map will explain the way Arizona DOT handles carriageways in the LRS network and the best practices and ideas around it. 

Carriageways are linear representations of a roadway that has width for vehicles to traverse.  Vehicles are not restricted by a barrier (i.e. median).  A carriageway typically is compressed of a number of lanes, shoulders and medians.

Carriageways are divided into two categories; Single and Dual. This is also referred to as undivided and divided roadways respectively and is used interchangeably at times. The basis of this distinction comes down to whether or not there is a passable median in a roadway. If there is a median that is larger than 4 feet and has an impassable feature, such as a gate or wall, that is considered a divided highway, or Dual Carriageway (*Add a photo example of a divided highway such as I-10 here). If there is only a painted median or a planter that could be driven over, this is an undivided highway or Single Carriageway (*Add a photo example of an undivided highway such as SR-87 in Payson here). 

A single carriageway greatly depends on the route’s functionality. This is an undivided roadway that is not physically divided. Whilst this means there is no positive barrier, there still could be a type of median that separates the different sides of the roadways. This could be a type of planter, or a sunken median that is passable. This is important to report as a single carriageway for many reasons but one is with E-911. This lets emergency vehicles know if they can pass over the median in any scenario. Another piece of information that is important is that the centerlines for non cardinal and cardinal routes can be visualized as a single carriageway with overlapping, concurrent routes. 

A dual carriageway is the opposite. This is a positive barrier that is at least 4 feet wide and presents what is called a positive barrier that would prevent a vehicle from passing it. This is also referred to as a divided highway. All Interstates must be divided, so they are also represented as a dual carriageway in the LRS. While the broad rule for what designates a dual carriageway is whether it is separated by a physical barrier, however, for E-911 purposes, roadways that may have planters or other types of medians may be considered single carriageways. This is to inform emergency vehicles if they can pass over a median of some kind.

With the knowledge of single and dual carriageways, now digging into the different types of carriageways is important. The different types of carriageways can be ramps, or frontage roads, or rest areas, but are still denoted in single or dual carriageways. As in, this Ramp, the example below is a Truck Escape Ramp, in a single carriageway.

The following are all the Carriageway Types:

TI Ramps

Rest Areas

Inspection Station Ramps

Weigh Station Ramps

Parking Area Ramps

Frontage Roads

Truck Escape Ramp

Weigh In Motion

Port of Entry

Maintenance Yard

Along with all of these, there is:

• Fuel Island
• Equipment Shop
• Mainline: SHS Mainline; I, S, or U
• Local: Non SHS or E-Ramp

As discussed previously, the linear representation and definition of carriageways is somewhat loose when it comes to FHWA guidelines. FHWA’s guidelines show that a divided highway is a multi-lane facility with a curbed or positive barrier or a median that is 4 feet or wider. Divided Highway is a multi lane facility with a curbed or positive median or a median that is 1.2 meters (4 ft) or wider. While states may adjust this definition as a way to designate a road as a dual carriageway, the blanket rule is that dual carriageways are specific to divided roadways. Divided highways must also be reported for both cardinal and non cardinal directions. Cardinal refers to the direction of increasing mileage.  In Arizona this is typically from West to East and from South to North, all though there are exceptions such as SR-85.

With the above in mind, there may be leniency on state DOT as to what they consider to be a dual carriageway, so a undivided roadway may be represented in dual carriageways if one feels it is necessary. Divided roadways may also be reported as an undivided roadway and not in a dual carriageway if a state DOT chooses as well. An example of this could be with a grassy median that extends 4 feet, but could be excluded as a divided roadway for 9-11/safety purposes.

Another implication of carriageways and HPMS is in the ownership reporting. Ownership according to the HPMS Field Manual in section 6.1, must be reported on all Federal Aid Highways in full extent. A Federal Aid Highway is defined as a public highway eligible for assistance under chapter Title 23, U.S.C, other than a highway functionally classified as a local road or rural minor collector. For LRS purposes, carriageway data should be reported independently for both directions of travel associated with a divided highway, for which dual carriageway GIS network representation is required per guidance from the HPMS Manual.

Because the carriageway distinction in the LRS varies between state to state, it is hard to narrow down best practices. Depending on the state DOT, each state has a best practice, however, there are a few practices that present themselves as being the most useful. 

• Single carriageways depend greatly on the route functionality, curbed medians and cut outs are allowed in single carriageway geometry. 
• A cardinal and non cardinal route can be represented by a single carriageway even though there are two routers travelling in opposite directions.
• Dual carriageways also depend on the route functionality, but must have a positive barrier that divides the routes. 
• Concurrent routes can result in overlapping routes. These are allowed in the LRS by leveraging route dominance. 
• Routes with gaps are permissible and are common in state and non state owned-routes. 
• Two way left turn lanes are delineated via painted median in events, this is represented by a single carriageway. 
• Traffic circles are displayed as divided roadways since they are only being one way directionally, this assists with the asset management. 
• Turn lanes and ramps that are considered grade separated intersections will not have an associated junction. The illicit a facility type change. 

Still having trouble understanding what carriageways are? Reach out to the Arizona Department of Transportation, Multi-Modal Planning Division for further explanation.

If you would like more information about the different variables required for HPMS and other transportation reporting, feel free to visit the  Data Analytics section website  or another related story map below:

 Certified Public Miles (CPM) 

 Traffic: How is it reported? 

 Highway Performance Monitoring System (HPMS) 

 Junctions 

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