DIAMOND INTERCHANGE VARIATIONS

VARIATIONS IN LANE LAYOUTS OF DIAMOND-STYLE INTERCHANGES

The design of the standard diamond interchange has been modified in various ways, to increase either capacity or flow performance. Most of these variations involve redirecting some movements to the left of other movements flowing the other way.

The demands of the design are complicated if the ramps also carry traffic on frontage roads, or are connected to rebound interchange ramps. There is no difference between the extra movements for frontage roads and the extra movements for a rebound interchange. Both need the variations shown on the right side of the diagram. But the rebound interchange also needs U-turn movements that are not shown on some of the diagrams.

Here are some of the designs that were tried, in the original design form, and with the frontage road flows added. The animations in the diagrams show the typical traffic signal phasing for each design:

Black indicates stopped movements.
Green indicates flowing movements.
Red indicates movements being stored in lanes for later release.

TABLE OF DIAMOND INTERCHANGE VARIATIONS

Without Frontage Roads	With Frontage Roads	Name	Author's Name Variation	nIS	nPH	Conflicts				Tangle			FR	Conditions
Without Frontage Roads	With Frontage Roads	Name	Author's Name Variation	nIS	nPH	DV	MR	XG	WV	XR	EN	EX	PN	Conditions
		1. STANDARD DIAMOND	STANDARD DIAMOND	2	3 (6)	6	6	6	0	0	2	2	0	Storage limits flow
		2. SAME, WITH FRONTAGE ROADS	WITH FRONTAGE ROADS	2	6 (3)	10	10	10	4	0	2	2	0	Storage limits flows
		3. CONTRAFLOW LEFT	BRAIDED ENTRANCE LEFTS	2	3	6	6	8	0	0	0	2	0	Storage limits exit flow
		4. SAME, WITH FRONTAGE ROADS	WITH FRONTAGE ROADS	2	3	10	10	10	4	0	0	2	0	Storage limits exit flow
		5. DIVERGING TURNS	BRAIDED LEFTS	6	3 (5)	8	8	6	2	0	0	0	0	Flows not limited
		6. SAME, WITH FRONTAGE ROADS	WITH FRONTAGE ROADS	2 (4)	4 (8)	12	12	14	6	0	0	0	1	Flows not limited
		7. DIVERGING DIAMOND	BRAIDED CROSS STREET	2	2	6	6	2	2	2	0	0	0	50 percent limit, Storage limits exit
		8. SAME, WITH FRONTAGE ROADS	WITH FRONTAGE ROADS	2	3	10	10	6	6	2	0	0	1	Storage limits exit
		9. HALF DIVERGING DIAMOND	SINGLE BRAID TURNS	4	3 (5)	7	7	4	2	0	0	0	0	Flows not limited
		10. SAME, WITH FRONTAGE ROADS	WITH FRONTAGE ROADS	4	4 8	11	11	10	6	0	0	0	1	Flows not limited
		11. SINGLE POINT URBAN (SPUI)	SPUI	1	3 (5)	6	6	8	0	0	0	0	0	Flows not limited, large bridge
		12. SAME, WITH FRONTAGE ROADS	WITH FRONTAGE ROADS	1	4 (8)	10	10	16	4	0	0	0	1	Flows not limited, large bridge
Without Frontage Roads	With Frontage Roads	Name	Author's Name Variation	nIS	nPH	DV	MR	XG	WV	XR	EN	EX	FR	Conditions
Without Frontage Roads	With Frontage Roads	Name	Author's Name Variation	nIS	nPH	Conflicts				Tangle			PN	Conditions

KEY:

nIS - Number of intersections in interchange (alternate layout in parentheses)
nPH - Number of phases for signals (alternate phase plans in parentheses)
DV - Number of diverging conflicts (includes frontage road - ramp diverges)
MR - Number of merging conflicts (includes frontage road - ramp merges)
XG - Number of crossing conflicts
WV - Number of weaving conflicts (includes frontage road - ramp weaves)
XR - Cross street movement tangles (places where cross-street movements cross each other)
EN - Entrance left turn tangles (places where entrance ramp left turns cross each other)
EX - Exit left turn tangles (places where exit ramp left turns cross each other)
FR PN - Penalty (in extra phase periods) for adding frontage roads.

COMPARING DIAMOND INTERCHANGE VARIATIONS

1. STANDARD DIAMOND

The distance between the intersections limits the storage of left turns when the 3-phase signal is used.
The 6-phase signal (see the frontage road diagram) removes the storage limits.
The bridge needs lanes in both directions, plus the turn storage lanes.

2. STANDARD, WITH FRONTAGE ROADS

The distance between the intersections limits the storage of left turns when the 3-phase signal (see the non-frontage diagram) is used.
The 6-phase signal removes the storage limits.
The bridge needs lanes in both directions, plus the turn storage lanes.
The frontage road adds no extra phases, but capacity for left turns is limited.
This interchange is the one most often used to connect to rebound interchanges.

3. CONTRAFLOW LEFT

Storage between the intersections limits the capacity of the exit ramps, but not the entrance ramps.
The entrance ramp left turn phases and opposing straight ahead phases can be split.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The bridge needs lanes in both directions, plus the turn storage lanes and three divider strips.

4. CONTRAFLOW LEFT WITH FRONTAGE ROADS

Storage between the intersections limits the capacity of the exit ramps, but not the entrance ramps.
The entrance left turn phases and opposing straight ahead phases can be split.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The bridge needs lanes in both directions, plus the turn storage lanes and three divider strips.
The frontage road adds no extra phases, but capacity for left turns is limited.

5. DIVERGING TURNS

Flows are not limited by the design.
The entrance ramp left turn phases and opposing straight ahead phases can be split.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The bridge needs lanes in both directions, plus the turn storage lanes and three divider strips.

6. DIVERGING TURNS WITH FRONTAGE ROADS

Flows are not limited by the design.
All left turn phases and opposing straight ahead phases can be split.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The interchange can be operated as a large 8-phase quad left turn intersection.
The bridge needs lanes in both directions, plus the turn storage lanes and three divider strips.
The frontage road adds extra phases.

7. DIVERGING DIAMOND

Exit ramp flows are limited by storage space.
The signals can be 2-phase.
Opposite directions on the cross street need different signal phases.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The bridge needs lanes in both directions. This is the smallest bridge of any plan.

8. DIVERGING DIAMOND WITH FRONTAGE ROADS

Exit ramp flows are limited by storage space.
The signals can be 3-phase.
Opposite directions on the cross street need different signal phases.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The bridge needs lanes in both directions. This is the smallest bridge of any plan.
The frontage road adds extra phases.

9. HALF DIVERGING DIAMOND

Flows are not limited by storage space.
The entrance ramp left turn phases and opposing straight ahead phases can be split.
The bridge needs lanes in both directions, plus the turn storage lanes and two divider strips.
The asymmetrical design can be confusing.

10. HALF DIVERGING DIAMOND WITH FRONTAGE ROADS

Flows are not limited by storage space.
All left turn phases and opposing straight ahead phases can be split.
The interchange can be operated as a large 8-phase quad left turn intersection.
The bridge needs lanes in both directions, plus the turn storage lanes and two divider strips.
The frontage road adds extra phases.
The asymmetrical design can be confusing.

11. SINGLE POINT URBAN (SPUI)

Flows are not limited by the design.
The entrance ramp left turn phases and opposing straight ahead phases can be split.
The bridge needs lanes in both directions, plus the turn storage lanes. But it must be larger to cover the ramp curves.
The frontage road adds extra phases.

12. SPUI WITH FRONTAGE ROADS

Flows are not limited by the design.
All left turn phases and opposing straight ahead phases can be split.
The left turn storage is greater for a lagging left, as opposed to a leading left.
The interchange is usually operated as a large 8-phase quad left turn intersection.
The bridge needs lanes in both directions, plus the turn storage lanes. But it must be larger to cover the ramp curves.
The frontage road adds extra phases.

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