DIRECTION SIGNS

Determination of Sight Distances

Main Roads WA

October1997

Traffic Engineering and Transport Planning Consultants

DIRECTION SIGNS

Determination of Sight Distances

Main Roads WA

October 1997

LMA Job Ref J9727RP1

L Millar & Associates, 8 Karo Place, Duncraig, Western Australia 6023 Ph (08) 9246 5359 Fx (08) 9246 4546

CONTENTS

1.INTRODUCTION1

2.THEORETICAL BACKGROUND2

3.PROPOSED GUIDELINES4

APPENDIX

Operational Guidelines

INTRODUCTION 1

Maintenance of the road verge is being undertaken under Term Maintenance Contracts. A need has been identified to determine the extent of pruning or clearing required to ensure that adequate visibility is achieved to direction signs.

The purpose of this report is to prepare operational guidelines that can be used to determine the required sight distances to direction signs and other large road signs.

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THEORETICAL BACKGROUND2

A literature review has been carried out into the determination of sight distances for direction signs. The most applicable information is from Guide to Traffic Engineering Practice, Part 8, Traffic Control Devices, Austroads, 1988. This reference has been used to develop the following theoretical background which has been used as the basis for the operational guidelines.

The sight distance (or legibility distance) to a sign is made up of three components shown in Figure 1:

(a)Observation distance - Distance travelled observing orscanning the sign.

(b)Reading distance - distance travelled while reading the sign legend.

(c)Lateral vision distance - distance from the sign at which the sign is outside the normal range of lateral vision.


Drawing modified from Figure 3.1 Guide to Traffic Engineering Practice, Part 8, Traffic Control Devices, Austroads, 1988.

Figure 1:Sign Legibility Distance

(a)Observation Distance

The driver does not start to read the sign until some time after the sign is first observed. This distance is generally considered to be about a third of the legibility distance to the sign. Therefore, the distance travelled while observing or scanning the sign is:

Observation distance = L/3

where:L is the legibility distance

(b)Reading Distance

The reading distance depends upon the speed at which the driver is approaching the sign and complexity of the sign. Research indicates that the time taken by an average observer to read a sign is a function of the number of words in the sign legend. For a sign containing 2 to 8 words (with the recommended letter size), the following relationship applies:

Reading distance = v (0.32N - 0.21) metres

where:v is the speed of the vehicle in m/sec

N is the number of words on the sign

(c)Lateral Vision Distance

A driver's normal range of lateral vision is limited to 10 degrees for a side mounted sign. Therefore, the driver should have completed reading the sign before the angle between the road and the centre of the sign exceeds 10 degrees. This distance in advance of the sign can be expressed as:

Lateral vision distance = S cot 10

where:S is the lateral offset from the driver to the centre of the sign

The legibility distance (L) can now be expressed as a sum of the following components:

L= Observation distance + Reading distance + Lateral vision distance

L = L/3 + v (0.32N - 0.21) + S cot 10 metres

2L/3= v (0.32N - 0.21) + S cot 10 metres

L= 3/2(v(0.32N - 0.21) + S cot 10) metres

L= v(0.48N - 0.32) + 8.51S

or if the approach speed is in km/h,

Legibility Distance = V(0.13N - 0.09) + 8.5S

where:V is approach speed in km/h

N is the number of words on the sign (for 2 to 8 words)

S is the lateral offset from the driver to the centre of the sign

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PROPOSED GUIDELINES3

The following theoretical expression has been derived:

Legibility Distance = V(0.13N - 0.09) + 8.5S

where:V is approach speed in km/h

N is the number of words on the sign (for 2 to 8 words)

S is the lateral offset from the driver to the centre of the sign

In practice, the legibility distance determined by this equation should be assumed to be an absolute minimum distance for clearing of vegetation to ensure clear sight line. The desirable sight distance should be increased to allow for the following factors:

  • subsequent vegetation growth after pruning; and
  • inadequate letter size on the sign.

The increase to achieve the desirable sight distance is assumed to be 50%.

The driver should be able to view the whole sign from the required sight distance. Therefore, verge maintenance is required when any part of the sign is obscured.

The definition of what constitutes a "word" is unclear. It is assumed that this should be any groups of letters or individual symbols in the sign legend (excluding arrows). Although the original equation for reading time was only up to 8 words, it would appear valid to extend the number of words to 10 to include the more complicated signs that have been installed.

The lateral offset (S) is determined for the sum of:

  • the distance from the driver to the kerb or edgeline (assumed to be 2 m);
  • the distance from the kerb or edgeline to the side of the sign (assumed to be an average of 2.5 m, although this could be as low as 0.5 m for a kerbed road);
  • half the width of the sign (assumed to be 1.5 m).

The total offset is therefore about 6m. The actual offset could be as low as 4 m and could be greater than 6 m for a sign on a road with wide shoulders. However, the sight distance is not particularly sensitive to the offset distance (17 m) and for practicalpurposes a single figure has been adopted.

Operational guidelines for the determination of desirable and absolute minimum sight distances are contained in the Appendix. The guidelines are intended for field use. They include a description on how to estimate the required sight distancesfrom the number of words in the sign legend and the vehicle speed.

1

APPENDIX

Operational Guidelines

OPERATIONAL GUIDELINE FOR THE DETERMINATION

OF SIGHT DISTANCES TO DIRECTION SIGNS

Vegetation in the road verge can often obscure the visibility of direction signsand other large road signs and must be maintained to ensure that adequatevisibility is available. These guidelines have been prepared to determine the extent of pruning or clearing of the vegetation that is necessary to achieve the required sight distances.

The sight distance to the direction sign is measured along the road from point at which the whole sign is visible to the driver. For multi lane roads, the sign is to be viewed from the lane closest to the road verge.


Figure 1:Sight Distance Measurement

Minimum sight distance requirementscan be determined in Figures 2 and 3 usingthe approach speed and the number of words or symbols (excluding) on the sign. Theapproach speed should be taken as the posted speed limit of the road or the normal operating speed on roads which have not been speed zoned.

The graph in Figure 2 should be used to determine the desirableminimum sight distance to the sign. If the measured sight distance is less than the minimum sight distance, treepruning or clearing of low vegetation is required.

Sight distances given in Figure 3 should only be used where the sight distance determined from Figure 2 cannot be achieved without extensive clearing or because the visibility of the sign is obscured by a road embankment. The sign should be relocatedif the minimum requirement in Figure 3 cannot be achieved.


Figure 2:DesirableMinimum Sight Distance Requirements


Figure 3:Absolute Minimum Sight Distance Requirements