Section 2: Curves and Turns
Anchor: #i1001159Introduction
Horizontal curves having a safe operating speed of 5 miles per hour or more below the posted maximum speed limit should be signed with advisory speed limits. Vertical curves may also be signed with advisory speed limits.
The speed to be posted will be based on results obtained from test runs in a vehicle equipped with either a mechanical ball-bank indicator (see Figure 5-2) or an electronic accelerometer, not the calculated value. (See following discussion of Calculated Speed. )
Figure 5-2. Two types of ball-bank indicators
Anchor: #CHDBFBICCalculated Speed
For curves and turns, the calculated speed is to be used as a guide for making the initial test run and as a check on the speed obtained by the use of the mechanical or electronic bank indicator. The calculated speed is not, however, to be used as the sole basis for selecting the posted speed. (See Selecting Speed for Posting at the end of this section for additional discussion.)
Calculate the design speed of the curve under consideration using the formula
where:
V = vehicle speed in miles per hour
R = radius of curve in feet
e = rate of roadway superelevation in feet per foot
f = side friction factor with value depending on speed as follows:
|
|
If the speed is: |
Then the side friction factor is: |
|---|---|---|
|
0.15 |
|
|
0.18 |
|
|
0.31 |
Anchor: #i1001236
Selection of Car and Mounting of Bank Indicator
Select an average passenger car for making the test runs and mount the bank indicator on the center line of the dash. Suitable metal strap mountings can be made on which to mount the indicator, as shown in Figure 5-3, Suggested mounting for ball-bank indicator on dash. The metal strap holding the right-hand side of the indicator on dash mountings should be slotted and a thumb nut provided so the steel ball can be adjusted to the zero degree position by raising or lowering the right side of the indicator. If there is any doubt about the selection of an average car, a bank indicator should be mounted on three different makes or year models for a check.
Figure 5-3. Suggested mounting for ball-bank indicator on dash
Anchor: #i1001258Before Conducting Test Runs
To ensure proper operation of the bank indicator, it is critical that the following steps be taken before conducting test runs:
- Inflate all tires to the uniform pressure used during speedometer calibration.
- Calibrate the test vehicle’s speedometer (see following procedure)
- Zero the bank indicator (see following procedure).
Calibrating Speedometer
It is essential that speedometers be calibrated accurately so that advisory speeds will be uniform throughout the state.
Calibrate the speedometer for recording of speed. The accuracy of the odometer for recording distance should be checked against a measured distance. Calibration for speed can be done easily with a radar speed meter or by timing the car over a measured distance. The speedometer should be checked for each 5-mile-per-hour interval over 20 miles per hour, and several test runs should be made for each speed so that an average value may be obtained.
Anchor: #i1001298Zeroing the Bank Indicator
The bank indicator must be adjusted to the zero reading before test runs are conducted. This must be done with the car straddling the center line of the pavement on a tangent section to give the effect of a flat level surface and the driver and recorder in the same position in which they will ride during the test runs.
It is essential that the driver and recorder be in the same position when the bank indicator is set to the zero reading as they will be when the test runs are made, because a shift in the load is reflected in a change of the indicator reading.
Anchor: #i1001313Conducting Bank Indicator Test Runs
The curve should be driven at the calculated speed described previously on the initial run. If the calculated speed is not available, the curve should be driven at an estimated speed approximately 5 miles per hour slower than that which the driver feels they can maintain throughout the entire length of the curve.
Each succeeding run should be made at a speed 5 miles per hour greater than the preceding run until the bank indicator reading reaches:
- 10 degrees for speeds of 35 mph or more
- 12 degrees for speeds of 25 and 30 mph
- 14 degrees for speeds of 20 mph or less.
On each test run, the driver should reach the test run speed at a distance of at least 0.25 mile from the beginning of the curve and maintain this speed throughout the entire length of the curve. The path of the car throughout the curve should be maintained as nearly as possible in the center of the right hand lane.
Test runs should be made in each direction on each curve.
On each of the speeds of the test runs, the observer must carefully note the position of the ball throughout the length of the curve and record the maximum deflection in degrees. The readings should be interpolated as closely as possible to the nearest degree.
Anchor: #i1001358Alternate Ball-Bank Indicator Test Run Method
An alternate procedure, intended to minimize the number of test runs required to determine the speed for which the curve is to be posted, is as follows:
- Drive the car at a speed of 5 miles per hour around the curve, staying as nearly as possible in the center of the right hand lane, and record the maximum deflection of the ball-bank indicator in degrees. Record the reading of the ball-bank indicator as plus if the deflection of the ball is to the right on a right hand curve and to the left on a left hand curve. The reading of the ball-bank indicator should be recorded as minus if the deflection of the ball is to the left on a right hand curve and to the right on a left hand curve.
- Drive around the curve at a constant speed that can be maintained without acceleration or deceleration and without driving outside the right hand traffic lane. Record the maximum deflection of the ball-bank indicator and the speed at which the curve was driven.
- Compute the maximum safe speed for the curve by solving
the following formula for V2:
NOTE: For difficulties viewing below table, see PDF.
Anchor: #i1005332 where:
θ1
= maximum ball-bank indicator reading as recorded in Step 2
θ2
= 10° for 35 mph and greater or
12° for speeds of 21 through 34 mph or
14° for 20 mph and less
φ
= ball-bank indicator reading as recorded in Step 1
V1
=vehicle speed in miles per hour at which 1 was recorded in Step 2
V2
=calculated safe speed in mph for which curve is good.
- Drive around the curve, staying in the right-hand lane,
at the calculated speed V2 without acceleration
or deceleration as a check on measurements and computations. The
ball-bank indication recorded at speed V2 should
then be:
- 10° for speeds of 35 mph or more
- 12° for speeds of 25 and 30 mph
- 14° for speeds of 20 mph or less.
Selecting Speed for Posting
Remember, the speed to be posted shall be based on the results obtained from test runs with the ball-bank indicator (described later in this section), not the calculated value.
The posted speed shall be a multiple of 5 miles per hour.
In selecting the speed to be posted, care should be taken that the calibrated speed for any given speedometer reading is used rather than the speedometer reading itself. As a final check, the posted speed is aimed at the highest value that will permit the average car to travel around the curve in its own lane without causing an uncomfortable side throw to its passengers. The speed to be posted on the curve should not be reduced arbitrarily below that determined by the procedures provided in this section.
When there is a reverse curve or a series of three or more curves, the advisory speed sign shall show the value for the curve having the slowest safe speed in the series.