Traffic Monitoring Guide

Appendix K. TYPICAL INTERCHANGE AADT ESTIMATION

 

The following are equations and example computations of using relevant data to compute unknowns rather than field counting for the most common interchange configurations.

Source: Federal Highway Administration.

FIGURE K-1. TYPICAL DIAMOND INTERCHANGES

 

Mathematical Formulas

 

(1)

 

(2)

 

(3)

 

(4)

While formulas 1 and 3 require cross street data, ramps 1 and 3 can be counted and formulas 2 and 4 can be used to estimate ramps 2 and 4.

Example

A diamond interchange located on an E/W freeway has directional mainline data both upstream and downstream of the interchange. Two ramps need to be counted in order to use formulas (3) and (5). Ramps R1 and R4 were counted and the following data are now known:

 

M1E	=	25,000	R1	=	1,200
M2E	=	23,200	R4	=	2,350
M1W	=	31,000
M2W	=	30,000

Source: Federal Highway Administration.

FIGURE K-2. DIAMOND INTERCHANGE RAMP ESTIMATION PROBLEM

 

R2	=	(M1W - M2W ) + R1
R2	=	(31,000 - 30,000) + 1,200
R2	=	2,200
R4	=	(M2E - M1E ) + R3
2,350	=	(23,200 - 25,000) + R3
2,350	=	-1,800 + R3
R3	=	4,150

 

Source: Federal Highway Administration.

FIGURE K-3. TYPICAL TRUMPET AND THREE-LEGGED DIRECTIONAL INTERCHANGES

 

Mathematical Formulas

While formulas 5 and 8 require cross street data, ramps 2 (or loop 1) and 4 can be counted and formulas 6 and 7 can be used to estimate ramps 1 and 3.

Example

A trumpet interchange located on an E/W freeway has directional mainline data both upstream and downstream of the interchange. Two ramps need to be counted in order to use formulas (9) and (10). Ramps R1 and R4 were counted and the following data are now known:

 

M1E	=	21,000
M2E	=	19,300
M1W	=	16,500
M2W	=	18,900
R1	=	2,800
R4	=	2,650

Source: Federal Highway Administration.

FIGURE K-4. TRUMPET INTERCHANGE RAMP ESTIMATION PROBLEM

 

R1	=	(M2W - M1W ) + L1
2,800	=	(18,900 - 16,500)+ L1
2,800	=	2,400 + L1
L1	=	400
R3	=	(M1E- M2E ) + R4
R3	=	(21,000 - 19,300) + 2,650
R3	=	4,350

Source: Federal Highway Administration.

FIGURE K-5. TYPICAL CLOVERLEAF INTERCHANGE

 

Cloverleaf interchanges are the most complex and data intensive scenario for volume to ramp count relationships.

Formulas (9) through (12) can be used directly assuming some combination of mainline, cross street, and ramp volumes are known for a given year.

A weight factor does not need to be used for exit ramps when approaching the cross street because vehicles do not have an option of which direction to take once on a ramp.

Mathematical Formulas

Mainline and cross-street AADTs available with one ramp known.

• In order to calculate all ramps, two ramps from each of the following groupings need to be counted in total:
  

  {R₁ , L₁ , R₂ , L₂ }

  {R₃ , L₃ , R₄ , L₄ }

  For example, if R₁ is already known, then count L₁, R₂, or L₂ plus two ramps from the second group.

• Once volumes are known for four ramps, use formulas (9) through (12) to determine the remaining volumes.

If only mainline AADT data are available, count three ramps from each of the following lists:

{R₁ , L₁ , R₂ , L₂ }

{R₃ , L₃ , R₄ , L₄ }

With six ramps counted, use formulas (9) and (11) to determine the volumes for the remaining ramps.

Example

A cloverleaf interchange located at an intersection of two freeways has directional mainline (E/W) data both upstream and downstream of the interchange. Two ramps need to be counted in order to use formulas (9) and (11). Ramps R₁ and R₄ were counted and the following data are now known:

M1E	=	54,000
M2E	=	51,500
M1W	=	58,500
M2W	=	59,000
R1	=	2,500
L1	=	2,100
R2	=	2,800
R3	=	2,200
L3	=	2,450
R4	=	2,500

Source: Federal Highway Administration.

FIGURE K-6. CLOVERLEAF INTERCHANGE RAMP ESTIMATION PROBLEM

 

R₁ = (M_2W - M_1W ) + (L₁ - L₂ ) + R₂

2,500 = (59,000 - 58,500) + (2,100 - L₂ ) + 2,800

2,500 = 5,400- L₂

L₂ = 2,900

 

R3 = (M_1E - M_2E ) + (L₃ - L₄ ) + R₄

2,200 = (54,000 - 51,500) + (2,450 - L₄) + 2,500

2,200 = 7,400 - L₄

L₄ = 5,250