Erection of clearance signs#
This section provides advice on signing requirements at locations with restricted vertical clearances. The Heavy Vehicle (Mass, Dimension and Loading) National Regulation nominates the height restrictions for vehicles.
These limits may be exceeded only with the prior written permission of the Superintendent of Traffic (Queensland Police Service) or performance guidelines issued by the Chief Executive (Transport and Main Roads) and subject to compliance with any conditions of such permission or guidelines. The limits apply to the motor vehicle, together with its loading and equipment.
The signing requirements at underpasses are set out in Part 2 of the Queensland Manual of Uniform Traffic Control Devices. Clearances are specified in metres to one decimal place.
To determine the appropriate clearance height to be shown on the sign, the minimum clearance above the carriageway (or lane as appropriate) is measured to two decimal places, and then rounded to the nearest 0.1 m below the measured height. A carriageway is that portion of the road devoted particularly to the use of vehicles, inclusive of shoulders and auxiliary lanes.
In determining the minimum clearance, care must be taken to measure it at the correct point, taking account of road cross fall and bridge grade and geometry. On two-way undivided carriageways, only one value for minimum clearance is to be posted. On divided carriageways, the minimum clearance applicable to each direction is to be posted for viewing on the approach side only. It may also be necessary to allow for a sag vertical alignment correction.
Where it is necessary to make allowance for any sag curvature in the roadway under the structure, the following table can be used to determine the amount by which the measured clearance should be reduced because of the curvature. In such cases, the procedure is:
- a)measure minimum height clearance to two decimal places
- b)read the sag correction, using Table 6.5 following – subtract the sag correction from the measured minimum clearance, and
- c)round down to nearest 0.1 m below this figure.
| Minimum radii of sag V.C. (metres) | Sag correction (metres) 1. All routes except over-dimension vehicle routes | Sag correction (metres) 2. Over-dimension vehicle routes |
|---|---|---|
| 800 | 0.01 | 0.03 |
| 500 | 0.02 | 0.05 |
| 300 | 0.04 | 0.09 |
| 200 | 0.06 | 0.13 |
| 130 | 0.09 | 0.20 |
| 70 | 0.16 | 0.37 |
In the following situations, it will be necessary to measure the sag correction using a tape 9.5 m long (14.4 m for an over-dimension vehicle route) stretched tightly between longitudinal points on the road surface (the correction is the maximum height of the tape above the road surface):
- a)the radius of the vertical curve is unknown
- b)the radius is less than 70 m, and
- c)the vertical alignment under the structure is a combination of curves and straights.
No allowance is made in the signing for changes in clearance, which may occur from resealing or resheeting of the pavement. Attention to signs should be made if a reseal or resheet reduces the clearance by more than the rounding in previous information on vertical clearance measurement.
Correct clearance must be displayed at all times. In the case of roadworks, a check on the exact clearance should be included in the works:
- new construction – prior to opening to traffic for the first time, and
- resurfacing – prior to reopening to traffic.
No allowance on the signed clearance is made for variations in vehicle height due to vehicle load or atmospheric conditions. It is considered that operators should allow for such variations.
