In Alternative Design, the Adjustment Factor may be presented as a weighted value for site characteristics listed below. This approach allows users to understand the overall impact of a proposed change on the facility, rather than viewing separate Adjustment Factors for each crash type and severity.

When multiple crash predictions are present and vary by crash type and severity, Alternative Design calculates a weighted average Adjustment Factor. Although the Adjustment Factor for individual crash types and severities may be higher, the weighted value accounts for all crash types/severities and displays a single representative value.

Weighted adjustment factor equation used in the Alternative Design application:

The following adjustment factors for the specified roadway or intersection type will have the weighted Adjustment Factor (using the equation above) displayed in-app:

Segments

Freeway Segment (ISAT-e)

Adjustment Factor	Crash Type	Severity
Inside Shoulder	All	FI, PDO
Median Width	MV, SV	FI, PDO
Lane Width	MV, SV	FI
Horizontal Curve	MV, SV	FI, PDO
Outside Shoulder Width	SV	FI, PDO
Shoulder Rumblestrips	SV	FI
Outside Barrier	SV	FI, PDO
Outside Clearance	SV	FI
High Volume	MV, SV	FI, PDO
Median Barrier	All	FI, PDO
Lane Change	MV	FI, PDO

Speed Change Lanes

(Note: users must add a speed change lane as a related segment in the project selection page)

Adjustment Factor	Crash Type	Severity
Horizontal Curves	All	FI, PDO
Lane Width	All	FI
Inside Shoulder Width	All	FI, PDO
Median Width	All	FI, PDO
Median Barrier	All	FI, PDO
High Volume	All	FI, PDO
Ramp Entrance/Exit	All	FI, PDO

Ramp Segments and Collector/Distributor (CD) Segment

Adjustment Factor	Crash Type	Severity
Lane Width	MV, SV	FI
Right Shoulder Width	All	FI, PDO
Left Shoulder Width	All	FI, PDO
Right Side Barrier	All	FI, PDO
Left Side Barrier	All	FI, PDO
Weaving (CD-Segment Only)	All	FI, PDO
Lane Add / Drop	MV, SV	FI
Ramp Speed Change Lane	MV	FI
Horizontal Curves	MV, SV	FI, PDO

Intersections

Ramp Terminal (ISAT-e)

Adjustment Factor	Traffic Control	Crash Type	Severity
Exit Ramp Capacity	Ramp Stop Controlled	All	FI
Exit Ramp Capacity	Signalized	All	FI
Channelized Right-Turn Lane on Crossroad	Signalized	All	FI, PDO
Crossroad Left Turn Lane	All	All	FI, PDO
Crossroad Right Turn Lane	All	All	FI, PDO
Access Point Frequency	Ramp Stop Controlled	All	FI
Access Point Frequency	Signalized	All	FI, PDO
Segment Length to Adjacent Intersection	Ramp Stop Controlled	All	FI
Segment Length to Adjacent Intersection	Signalized	All	FI, PDO
Protected Left-Turn Lane Operation	Signalized	All	FI, PDO
Channelized Right-Turn Lane on Exit Ramp	Signalized	All	FI, PDO
Non-Public Street Leg	Signalized	All	FI, PDO
Skew Angle	Ramp Stop Controlled	All	FI
Median Width	Ramp Stop Controlled	All	FI
Median Width	Signalized	All	FI, PDO

Example Problem

How to calculate a weighted average Adjustment Factor for a 6-Lane Urban Freeway with the following site characteristics:

AADT = 110,000 vehicles per day
Segment Length = 1.0 miles
Lane width of 11 feet

MV = Multi-vehicle

SV = Single-vehicle

PDO = Property damage only

FI = Fatal injury

AADT = Annual Average Daily Traffic

L = Segment length

Exp = Euler's number

Safety Performance Function formula:

Where,

SPF Coefficients for Urban 6-Lane Freeways

Crash Type and Severity	a	b	c
MV FI	-5.587	1.492	0.001
SV FI	-2.055	0.646	0.001
MV PDO	-6.809	1.936	0.001
SV PDO	-2.274	0.876	0.001

Utilizing the AADT, Segment Length, and SPF Coefficients, the Predicted Crash Frequency by crash type (MV, SV) and Severity (FI, PDO) are as follows:

To calculate the Adjustment factor for Lane Width, use the following equations and table of coefficients:

Where:

W is the lane width

Coefficients for Lane Width Adjustment Factor

Crash Type and Severity	a	b
MV FI	-0.0376	0.963
SV FI	-0.0376	0.963

*Note: MV PDO and SV PDO crashes are not impacted by lane width. Therefore, the adjustment factor is assumed to be 1.0.

To calculate the weighted average Adjustment Factors, use the following formula (same as above):

The weighted average Adjustment factor can then be calculated based on the adjusted and the unadjusted predicted crash frequency values using the following equation.

The result of 1.01 will be displayed in the application to better understand the holistic impact of an 11-foot lane on total predicted crash frequency.

Alternative Design: Weighted Adjustment Factors