U-turn policies

U-turn policies can be set at the level of the network location, network analysis layer, or network dataset. The following subsections introduce various ways U-turns can be allowed or prohibited.

Curb approach

CurbApproach is a property on network locations, such as stops in route analysis layers and orders in vehicle routing problems. It specifies the direction a vehicle may arrive at and depart from the network location.

The CurbApproach property is accessed through a network location's Properties dialog box.

Learn how to access the Properties window of a network location

There are four values (for simplicity, these descriptions refer to stops instead of network locations):

Setting	Coded value	Description
Either side of vehicle	0	The vehicle can approach and depart the stop in either direction, so a U-turn is allowed. This setting can be chosen if it is possible and desirable for your vehicle to turn around at the stop. This decision may depend on the width of the road and the amount of traffic or whether the stop has a parking lot where vehicles can pull in and turn around. All arrival and departure combinations are allowed with the Either side of vehicle curb approach.
Right side of vehicle	1	When the vehicle approaches and departs the stop, the stop must be on the right side of the vehicle. A U-turn is prohibited. The allowed arrival and departure combination for the Right side of vehicle curb approach.
Left side of vehicle	2	When the vehicle approaches and departs the stop, the stop must be on the left side of the vehicle. A U-turn is prohibited. The allowed arrival and departure combination for the Left side of vehicle curb approach.
No U-Turn	3	When the vehicle approaches the stop, the stop can be on either side of the vehicle; however, when it departs, the vehicle must continue in the same direction it arrived in. A U-turn is prohibited. The allowed arrival and departure combinations for the No U-Turn curb approach.

The CurbApproach property was designed to work with both kinds of national driving standards: right-hand traffic (United States) and left-hand traffic (United Kingdom). First, consider a stop on the left side of a vehicle. It is always on the left side regardless of whether the vehicle travels on the left or right half of the road. What may change with national driving standards is your decision to approach from the right or left side. For example, if you want to arrive at a stop and not have a lane of traffic between the vehicle and the stop, you would choose Right side of vehicle in the United States but Left side of vehicle in the United Kingdom.

With right-hand traffic, the curb approach that leaves the vehicle closest to the stop is Right side of vehicle.

With left-hand traffic, the curb approach that leaves the vehicle closest to the stop is Left side of vehicle.

Since CurbApproach is a property on network locations, a network analysis layer can have mixed curb approaches among its objects.

Caution:

Currently when a network location is on a junction, Network Analyst allows U-turns there, even when its CurbApproach property is set to No U-Turns and the U-turns at Junctions property of the network analysis layer is set to Not Allowed.

U-turns at junctions

U-turns at Junctions is a property on network analysis layers that lets you restrict or permit U-turns at junctions that could occur during network traversal between stops.

Learn how to open the Layer Properties dialog box of a network analysis layer

The property has four settings:

Setting	Description
Allowed	U-turns are permitted at any junction. U-turns are permitted at junctions with any number of adjacent edges.
Allowed only at Intersections and Dead Ends	U-turns are prohibited at junctions where exactly two adjacent edges meet. U-turns are permitted at intersections (any junction with three or more adjacent edges) or dead ends (junctions with exactly one adjacent edge).
Allowed only at Dead Ends	U-turns are prohibited at all junctions, except those that have only one adjacent edge (a dead end). U-turns are permitted only at junctions with exactly one adjacent edge (dead ends).
Not Allowed	U-turns are prohibited at all junctions. Note, however, that U-turns are still permitted at network locations even when this setting is chosen; however, you can set the individual network locations' CurbApproach property to prohibit U-turns.

Reverse turns on the global turn delay evaluator

The global turn delay evaluator can be set up on any of the network dataset's time-based cost attributes. Since it is part of the network dataset, any network analysis layer solved on an impedance attribute that has an associated global turn delay evaluator is affected by the evaluator's current settings.

Learn how to open the Global Turn Delay Evaluator

You can use the global turn delay evaluator to generically penalize left, right, straight, and reverse turns at junctions. U-turns double back on the same edge as the approach. Reverse turns are similar to U-turns, but reverse turns are based on approach and depart angles and don't have to turn back on the same edge. The next graphics demonstrate this idea.

Here is a sharp turn at a street intersection. According to the settings of the Global Turn Delay Evaluator dialog box in the screenshot above, turns qualify as reverse turns if the angle of departure from the intersection is 30 degrees or less (60 degrees divided by two).

The reverse-turn wedge from the global turn delay evaluator is superimposed on the roads to show that the turn is sharp enough to qualify as a reverse turn.

If a similar turn is made on this intersection, it would not qualify as a reverse turn. Although the turn is sharp, it falls outside the reverse-turn range, and would qualify as a left turn instead.

U-turns double back on the same road as the approach, so they always qualify as reverse turns.

Assigning a prohibitively high penalty to reverse turns has the effect of restricting U-turns. However, the global turn delay evaluator can only penalize time-based impedance attributes, so if your impedance is distance based, for example, the global turn delay evaluator is not an option for setting a U-turn policy.

One benefit of the global turn delay evaluator is that if the network has hierarchies defined, penalties for reverse turns can vary based on the hierarchies of the roads that converge at a junction. For instance, making a reverse turn on a local road at an intersection with another local road could be lightly penalized, essentially allowing U-turns in those situations.

Learn more about global turns

Turn features with restrictions

Turn features are part of the network dataset; therefore, any network analysis layer that is linked to that network dataset is affected by active turn features.

Turn features with restrictions are useful if you want Network Analyst to prohibit U-turns only at specific locations, such as at intersections where U-turns are posted as illegal. Also, turn features offer the only method of restricting U-turns that traverse multiple edges, which are most often found at intersections with a divided road.

U-turns at specific junctions can be prohibited with turn features.

Turn features are the only method of restricting multipart U-turns, which are turns that traverse more than two edges.

Note that turn features supersede penalties from the global turn delay evaluator.

Learn more about turns_in_the_network_dataset