Appendix A: Guidelines for the installation of curve warning and advisory speed signs

General

Information on the treatment of curves is provided in treatment of curves. Curve warning signs, W12-1 and W12-2 series, combined with W12-3.2 curve sign supplementary advisory-speed signs should be installed on the approaches to horizontal curves when warranted by an Advisory Speed Assessment Survey.

Treatment of curves

Definitions

Advisory speed assessment survey

The measured advisory speed V_M is assessed by driving a test vehicle equipped with a ball bank gauge and a calibrated speedometer (or proven and accepted digital alternative) around the curve at a constant known speed V_O and observing the highest reading B, of the gauge. V_M is then obtained graphically from Appendix Figure 1 or by calculation, using the survey values. The rounded advisory speed V_R is also derived from  Appendix Figure 1. Each of nine permitted values of V_R, i.e. 15, 25, 35, 45, 55, 65, 75, 85 and 95km/h, is matched to a specific 10km/h range of V_M, e.g. V_R = 15km/h covers the range of V_M from 11 to 21km/h. The nine values of V_R are listed in Appendix Table 1 and shown on Appendix Figure 1; no other values of V_R should be used.

Appendix Table 1

Appendix Figure 1

At least two survey runs should be made in each direction for each curve because advisory speed values can differ by direction. Two runs in the same direction for each curve should produce the same result before that value can be adopted. If the two runs produce differing results a third run should be made to determine which of the previous results is appropriate.

Warrant for advisory speed signing

Only those curves that meet the warrant given in Appendix Table 1, also illustrated in Appendix Figure 2 should be signed for advisory speed. The criteria for this warrant are the rounded advisory speed V_R and the 85th percentile curve approach speed V₈₅. The 85th percentile curve approach speed V₈₅ is an estimate of the speed that only 15% of vehicles would exceed and it is assessed just before the point where vehicles start to slow for the curve.

Appendix Table 1

Appendix Figure 2

As a guide:

• the 85th percentile speed on long straights with ample seal width, excellent visibility and sound surface condition is typically about 10km/h higher than the speed limit. 85th percentile speeds drop below this as alignment and other road and roadside development factors become less than ideal; and
• the 85th percentile speed at the exit point of a previous curve can be assumed to be 20% higher than the rounded advisory speed for that curve.

Appendix Table 1 shows the warrant values required for the installation of advisory speed signs. These values are also illustrated in Appendix Figure 1 and Appendix Figure 2.

Appendix Figure 1

Appendix Figure 2

Appendix Table 1: Advisory speed sign warrant values

When the warrant for signing a curve is met, V_R becomes the curve advisory speed V_S, adopted for signposting. Removal of existing advisory speed signs may be considered if V_R does not meet the warrant and, in the opinion of the RCA, the sign has ceased to be of value.

Where advisory speed signing is not warranted, curve warning signs alone may be installed in situations where the curve is deceptive and not obvious to approaching drivers. Advisory speed signs should not be installed when V_M is greater than 101km/h, ie V_R would be 105km/h.

Appendix Figure 1: Determination of advisory speed on speed on horizontal curves

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Example in Appendix Figure 1: Observed survey speed, V_O, is 90km/h and ball bank reading, B, is 12. From the graph, or by calculation, V_M is approximately 84km/h. From the graph V_R is 85km/h and this is the value of the curve advisory speed, V_S, to be sign posted if the warrant given in Appendix Table 1 is met.

Appendix Table 1

Sign size

The size of curve warning and advisory speed signs should be increased as the degree of hazard increases. To determine the minimum size of a sign for a particular application refer to TCD manual Part 1, and Appendix Figure 2 and Appendix Table 2 of this manual.

TCD manual Part 1

Appendix Figure 2: Curve warning and advisory speed signs on horizontal curves - signing warrant and size determination

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Appendix Table 2: Minimum sign size

Sign location

A curve warning sign should be located where approaching drivers have an uninterrupted view of it over a distance of at least 120m in rural areas and at least 60m in urban areas and be in advance of the curve by at least the distance specified in Appendix Table 3.

Appendix Table 3: Location of curve warning and advisory speed signs

Isolated and adjacent curves

Isolated curves and adjacent curves should be signed as appropriate with one of the signs listed in Table 3‑1.

Table 3-1

Horizontal curve chevron sight board signs and chevron curve indicator sign

W20-1 chevron sight board - horizontal curve signs should only be installed on horizontal curves where the curve advisory speed is 15km/h or more below the 85th percentile operating speed, or where the hazard is such as to warrant Size D signs, as shown on Appendix Figure 2. The curve advisory speed shown on the curve warning sign must also be shown on the W20-1 signs.

Appendix Figure 2

W20-2 chevron curve indicator signs are normally used in conjunction with a W20-1 sign to further delineate the curve.

W20-2 signs may also be used on horizontal curves where the installation of a W20-1 sign is not warranted but, in the opinion of the RCA, some additional delineation is required throughout the curve, eg a long, large deflection angle, horizontal curve.