- It measures the overturning force (side friction), measured in degrees, on a vehicle negotiating a horizontal curve - whether it is an isolated curve, multiple "S" curve, or a ramp to/from a freeway.
- Typically used by the Department of Transportation and other advisory transportation agencies.
- Some FHWA research McHenry Software did with Jack Leisch & Associates (JEL) in the 80's was on highway curve design, etc and included an investigation of what the ball bank indicator actually measures.
- The “discomfort factor" is representative of the resultant acceleration that an occupant experiences during a recovery maneuver (Fig.4 see below).
- The use in the evaluation of cornering maneuvers of a hard mounted accelerometer measurement of lateral acceleration of the vehicle does not include the effects that the vehicle roll angle has on the occupant.
- For example, a vehicle in a normal cornering maneuver rolls in a direction opposite to the turn (I .e., rolls positive, or right for a left hand turn & visa versa) and therefore the “discomfort” which would be experienced is greater than the calculated lateral acceleration (i.e., V2/R).
- Curves are super-elevated to reduce or reverse the magnitude of the vehicle roll angle to reduce the “discomfort factor" felt by the driver and occupants.
- Many of the earlier geometric design criteria standards were based on experimental measures of a ball bank indicator during cornering maneuvers which essentially measured the same resultant acceleration as the HVOSM calculated "discomfort factor:
- The 'discomfort factor- output of tbe HVOSM illustrated the problems associated with cross-slope breaks on highway curves in HVOSM Studies of Highway Cross Slope Breaks on Highway Curves, which was also prepared as a part of the JEL study.
- In that study the problems associated with cornering on an adversely super-elevated curve (i.e.. cross-slope break) were demonstrated by examination of the “discomfort factor."
- Subsequent recommendations were made to reduce excursions onto the shoulder breaks which may result in unacceptable levels of driver “discomfort".
Sept 2021 update: replaced the figure with one at better resolution
- NOTE: the lateral component of gravity is Sin(PHI)
- where PHI is the vehicle roll angle/roadway slope angle
- and there may be a difference between the vehicle roll angle and roadway slope due to suspension response of the vehicle, but generally approximately the same
- NOTE2: for small angles, generally less than 15-20 degrees, the sine of the angle is approximately equal to the angle in radians
- The angle in radians is simply the angle divided by 57.29578
- see small angle approximations
- NOTE: the lateral component of gravity is Sin(PHI)