Ramp Count Estimation Procedures

Ramp Count Estimation Procedures

Daniel Jenkins, PEOffice of Highway Policy Information

October 24, 2012

Ramp Count Requirements

• Grade separated interchanges on Federal-aid highways– Except minor collectors and local (rural)– Except local (urban)

Ramp Count Study

• Develop mathematical formulas to estimate ramp volumes

• Typical interchanges– Diamond– Trumpet/3-leg directional– Cloverleaf

• Requires some ramp data counts

4

Algorithm Development• Volume relationships can be created for any interchange using this

concept• A directional downstream volume at a standard diamond interchange is

equivalent to:

• A cross street relationship exists as well requiring a weight factor for each ramp:

5

Diamond Interchange

Single Point Urban Interchange (SPUI)

6

Diamond and Single-Point Urban Interchanges

• Formulas (2) through (5) were derived to represent ramp volumes at a typical diamond interchange or SPUI

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Diamond and Single-Point Urban Interchange

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

– In this case, one additional ramp needs to be counted that is opposite to the ramp already known. Once this data is collected, formulas (3) and (5) can be used to determine the other two ramp volumes.

– Due to the complexity of the cross street relationships, these formulas involve all four ramps. They can be used to validate the volumes derived mined by using the two mainline equations.

8

Diamond and Single-Point Urban Interchange

• Mainline and cross-street AADTs available

– If only mainline and cross street count data is available, formulas (3) and (5) can be used once two opposite ramps are physically counted. The specific ramp combination does not matter as long as it is one of the following four combinations:

– Similar to the first scenario, the cross street equations can be used to validate the derived values from formulas (3) and (5).

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Diamond and Single-Point Urban Interchange

• Only mainline AADT data available (most typical situation)– In a scenario with less data, having only mainline counts requires two opposite

ramps to be counted in order to establish counts for all ramps. Potential permutations include:

– Once a permutation is counted, formulas (3) and (5) can be used to determine estimated counts for the remaining two ramps.

EXAMPLEA 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 is now known:

25000

31000 30000

23200

1200

2350

25000

31000 30000

23200

1200

2350

Trumpet Interchange

Three-leg Directional Interchange

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Trumpet and Three-leg Directional

• Using the relationships of mainline to ramp volumes and cross street to ramp volumes, formulas (8) through (11) were derived to represent ramp volumes at trumpet and three-leg directional interchanges.

• No weight factor was applied to R3 to determine how many cars travel towards C1.

– The amount of vehicles using this to u-turn is trivial compared to the total number of vehicles using that ramp.

15

Trumpet and Three-leg Directional

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

– No additional ramp counting is needed in this scenario. Formulas (8) through (11) can be used to solve for the remaining ramps.

16

Trumpet and Three-leg Directional

• Mainline and cross-street AADTs available

– If only mainline and cross street counts are available, formulas (8) through (11) can be used once on ramp is counted. It does not matter which ramp is counted.

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Task 3 – Trumpet and Three-leg Directional

• Only mainline AADT data available (most typical situation)

– In a scenario with less data, having only mainline counts requires two opposite ramps to be counted in order to establish counts for all ramps. Potential permutations include:

– Once a permutation is counted, formulas (9) and (10) can be used to determine estimated counts for the remaining two ramps.

EXAMPLEA 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 is now known:

18900

19300

16500

21000

2800

2650

18900

19300

16500

21000

2800

2650

If no mainline data exists, an origin/destination approach to the interchange yields two relationships in most instances (especially rural interchanges):

Alternate Estimate method for Diamonds/Trumpets/Three-Legged Interchanges

Cloverleaf Interchange

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Cloverleaf• Cloverleaf interchanges are the most complex and data intensive

scenario for volume to ramp count relationships. – Formulas (14) through (17) 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.

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Cloverleaf• Mainline and cross-street AADTs available with one ramp known

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

– 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 (14) through (17) to determine the remaining volumes.

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Cloverleaf

• Mainline and cross-street AADTs available

– If only mainline and cross street data is available, formulas (14) through (17) can be used once four ramps are counted.

– The four counted ramps must be diagonal to one another, i.e. one of the two groups of ramps below needs to be counted:

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Cloverleaf• Only mainline AADT data available (most typical situation)

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

– With six ramps counted, use formulas (14) and (16) to determine the volumes for the remaining ramps.

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EXAMPLEA cloverleaf interchange located at an intersection of two highway has directional mainline (E/W) data both upstream and downstream of the interchange. Six ramps need to be counted in order to use formulas (14) and (16). Ramps 1, 2, 3, and 4, and Loops 1 and 3 were counted and the following data is now known:

Cloverleaf Interchange

Cloverleaf Interchange

QUESTIONS?