To position power poles a safe distance from the road to minimise the likelihood of being accidentally hit by vehicles.

Transcription

1 Policy Statement Subject Placement of Rigid Distribution Poles Along Roads With Speed Limits Exceeding 70KM/H Approved by Robert Rogerson Signature & Date Distribution Standards and Policy Manager Authorised by Mark Wilshusen Manager Signature & Date Standards, Policy & Data Quality Issue Date 14 August 2006 Issue Initial Objective To position power poles a safe distance from the road to minimise the likelihood of being accidentally hit by vehicles. Context During traffic accidents, it is not uncommon for vehicles to leave the carriageway and collide with solid structures, such as poles. By positioning poles a safe distance from the road, this hazard can be reduced. References and Supporting Documentation - Main Roads website - Guide to the Relocation or Protection of Services. - Main Roads website - Guide to the Design of Road Safety Barriers. F012E9A1F265/0/utility.pdf - Main Roads website - Utility Providers Code of Practice. AS/NZS 3845:1999, Road safety barrier. Section 17 of the Rural Road Design, published by Austroads. Section 14 of the Urban Road Design, published by Austroads. Main Roads - WA roads with permanent traffic counters. Main Roads - Metropolitan roads with permanent traffic counters. Main Roads - South West roads with permanent traffic counters. Policy Statement Rigid power poles should not be installed within the Clear Zone. Policy Details For roads other than freeways or controlled access highways, the Main Roads requirements are defined in its Guide to the Relocation or Protection of Services, Section 4.3 as follows. Policy Statement 1

2 Where practical: Services should be placed on the alignment given in Appendix B of the Utility Providers Code of Practice. Above-ground services should be placed outside the Clear Zone, as detailed in the Guide to the Design of Road Safety Barriers, so as not to pose a hazard to motorists. The Guide to the Design of Road Safety Barriers, Section 2, states that a Clear Zone should be determined in accordance with Austroads Urban Roads Design Guide to the Geometric Design of Major Urban Roads, and Rural Road Design Guide to the Geometric Design of Rural Roads. The Clear Zone is defined in AS/NZS , Section 1.4.9, as - "the horizontal width of space available for the safe use of an errant vehicle which consists of the verge area and is measured from the nearside edge of the left-hand traffic lane. In the case of a divided road, it is also measured from the offside edge of the right-hand traffic lane to the edge of the pavement for opposing traffic. NOTE: This area may consist of a shoulder, a recoverable slope, a non-recoverable slope and a runout area, but all parts can be traversed. The desirable width is dependent on traffic volumes, speeds and the geometry of the road. Figure 1: Clear Zone layout The width of the Clear Zone may be limited by the presence of physical objects that limit or prevent errant vehicles from leaving the carriageway (e.g. a barrier rail or trees). The process for defining the width of the Clear Zone involves the following three steps: 1. Determining the Clear Zone width for straight roads without steep embankments. 2. Adjusting the width of the Clear Zone to accommodate curves. 3. Adjusting the width of the Clear Zone to accommodate steep embankments. Policy Statement 2

3 1. Determining the Clear Zone width for straight roads with flat embankments Figure 2: Clear Zone widths on straight roads with flat embankments (Published by Austroads) The Clear Zone width for straight roads without steep embankments can be determined from Figure 2, above. To use the diagram we need to identify two parameters: the design speed and the one-way Average Annual Daily Traffic (AADT). For most rural roads and roads managed by local governments, AADT values are below For major highways, they generally exceed The accurate AADT values can be obtained by contacting Main Roads Customer Services section, by sending an to the following address: Main Roads measures AADT by using both permanent and temporary count sites. The requesting AADT information must include a permanent count site reference number (shown on the Main Roads maps) or alternatively must clearly describe the section of the road for which information is required. It is necessary to obtain or estimate the AADT value and to calculate the corresponding Clear Zone width for the future. This is because the installed poles will remain in place for a long period of time. As a minimum, in accordance with the American Association of State Highway and Transportation Officials (AASHTO) guidance, we should estimate AADT values and Clear Zone widths for five years time. The future AADT values should be estimated by using a linear extrapolation method, demonstrated in Example 1. Example 1 Main Roads provided the following AADT values: last year vehicles per day, this year vehicles per day. What will be the value of AADT in five years time? The AADT growth rate is equal to: ( ) / 750 = , i.e. ~6%. In five years time, the AADT value will increase by ( ) 5 = = times. The future AADT will equal: 800 x = 1070 vehicles per day. Therefore, the AADT value to be used for calculating the Clear Zone width should be Policy Statement 3

4 After deciding on the AADT value, the width of the Clear Zone can be determined from Figure 2 by taking the following steps: Draw a vertical projection from speed value on the horizontal axis, up as far as the speed curve for the appropriate AADT value. Then draw a horizontal projection from that point on the speed curve towards the vertical axis and identify value of the Clear Zone width. Example 2 For a speed limit of 95km/h, and an AADT of 3000 vehicles per day, the value of Clear Zone width is 7.5m. Adjusting the width of the Clear Zone to accommodate curves Vehicles travelling along curves have a greater likelihood of leaving the carriageway and will travel a greater distance into the Clear Zone than when travelling along a straight line. Therefore, when power poles are located along the outside curve of a road, it is necessary to apply an adjustment factor to the Clear Zone width, calculated from Figure 2. Its value will depend on the curve radius and on vehicle speed, and can be determined from Figure 3. Figure 3: Adjustment factors for Clear Zones on curves (published by Austroads) The radius of a road curve can be estimated from aerial photos, available from NMS or from DFIS. For vehicle speed, use the speed limit applicable to the road. Example 3 On a curve with a radius of 500 metres and speed limit of 95km/h, the adjustment factor is 1.3. Therefore, if this curve is located on a road with an AADT of 3000 vehicles per day, as in Example 2, the width of the Clear Zone (CZ) should be multiplied by 1.3, as follows: CZ = 7.5 x 1.3 = 9.75 metres. Policy Statement 4

5 2. Adjusting the width of the Clear Zone to accommodate embankments. Embankments can have cut or fill slopes, as shown in Figure 4. Figure 4: Pole setback for cut/fill slope embankments. Fill slope embankments Fill slopes occur when a road is built higher than the adjacent land. They are divided into three categories: a) Recoverable slopes, with gradients of up to 1:6 (that is, one length of vertical distance to six lengths of horizontal distance). It is assumed that on these slopes errant vehicles would be able to slow down or manoeuvre to avoid poles, as they would on a flat surface. b) Partially recoverable slopes, with values between 1:5.5 and 1:3.5. Errant vehicles may not be able to reduce speed when travelling down these slopes. c) Non-recoverable slopes, steeper than 1:3.5. It is likely an errant vehicle would run uncontrolled down these slopes. a) Recoverable fill slopes CZ = the Clear Zone width determined from Figure 2 and adjusted for road curvature based on Figure 3. Figure 5: Recoverable fill slopes. Policy Statement 5

6 B For fill slope embankments with a ratio of 1:6 or less, as shown in Figure 5, the Clear Zone width is calculated as for flat embankments. b) Partially recoverable fill slopes ECZ - is the effective Clear Zone width, i.e. the Clear Zone width calculated for flat embankments adjusted for embankment slope. W W 2 - is the width from edge of thoroughfare lane to hinge point. is the embankment width. is the width from toe of embankment (minimum 3m). Figure 6: Partially recoverable fill slopes When a fill slope is classified as partially recoverable, half of the embankment width should be included in the Clear Zone width calculation. The calculations vary, depending on the width of the embankment. The formulas are given in Table 2. Clear Zone calculations for non-recoverable fill slopes are demonstrated in Examples 4 and 5. Example 4 If CZ = 7.5m, W 1 = 1m and B = 16m (wide embankment). CZ W1 <, as CZ W 1 = 6.5 and = W 2 = CZ W 1 B / 2, W2 = = 1.5m, W 2 is less than 3m so must be increased ECZ = W (CZ W 1 ), W 2 = 3m ECZ = x (7.5 1) = 14m Example 5 If CZ = 7.5 m, W 1 = 1m and B = 4m (narrow embankment). CZ W1 >, as CZ W 1 = 6.5 and = W 2 = CZ W 1 / 2, W B 2 = = 4.5m, (W 2 > 3m) Policy Statement 6

7 ECZ = W 1 + / 2 + W B 2, ECZ = = 7.5 m Policy Statement 7

8 Table 2: Effective Clear Zone formulas for partially recoverable fill slopes (values in metres) Slope type Formula description Formulas Partially recoverable fill slope Testing of embankment width Calculated distance from toe of embankment to edge of ECZ (W 2 ) Testing of W 2 for the minimum value of 3 metres Effective Clear Zone (ECZ) width Clear Zone distance (CZ) minus shoulder width (W 1 ) smaller than half of the embankment width ( ) CZ W < 1 2 Clear Zone distance (CZ) minus shoulder width (W 1 ) greater than half of the embankment width ( ) CZ W > N/A W 2 = CZ W N/A W 2 < 3 W 2 > 3 ECZ = W1 + 2 (CZ - W 1 ) W B ECZ = W W B ECZ = W W2 Policy Statement 1

9 B c) Non-recoverable fill slopes ECZ - is the effective Clear Zone width, that is, CZ width calculated for flat embankments adjusted for embankment slope W is width from edge of thoroughfare lane to hinge point is embankment width W 2 - is calculated distance from toe of embankment to edge of ECZ (minimum 3 metres) Figure 7: Non-recoverable fill slopes. When a fill slope is classified as non-recoverable, the embankment width ( ) B is excluded from the Clear Zone calculation. The calculations vary, depending on the width of the road shoulder, W1. The formulas are given in Table 3. Table 3: Effective Clear Zone formulas for non-recoverable fill slopes Slope types Nonrecoverabl e fill slopes Formula description Calculated distance from toe of embankment to edge of ECZ (W 2 ) Testing of W 2 for the minimum value of 3 metres Effective Clear Zone (ECZ) width in metres Formulas W 2 = CZ W 1 W 2 < 3.0 W 2 > 3.0 ECZ = W 1 + B + 3 ECZ = W 1 + B +W2 Clear Zone calculations for non-recoverable fill slopes are demonstrated in Examples 6 and 7. Example 6 If CZ = 7.5m, W 1 = 1m (narrow road shoulder) and B = 4m. W 2 = CZ W 1, W 2 = = 6.5m, (W 2 > 3m) ECZ = W 1 + B + W2, ECZ = = 11.5m Example 7 If CZ = 7.5m, W 1 = 5m (wide road shoulder) and B = 4m. Policy Statement 1

10 W 2 = CZ W 1, W 2 = = 2.5 m W 2 is less than 3m so must be increased ECZ = W 1 + B + W2, ECZ = = 12m W 2 = 3m Cut slope embankments Cut slopes are created when a road is built by cutting into the ground. They are divided into two categories: a) Traversable cut slopes Cut slopes flatter than 1:2 or with a height of less than 1.2 metres (regardless of slope) are classified as traversable. For these slopes the Clear Zone width should be calculated as for a flat embankment. No adjustment is required. CZ - is the Clear Zone width determined from Figure 1 and adjusted for road curvature based on Figure 2. Figure 8: Traversable cut slopes b) Partially traversable cut slopes Cut slopes both steeper than 1:2 and higher than 1.2 metres are classified as partially traversable. ECZ - W 1 - is the effective Clear Zone width, i.e. CZ width calculated for flat embankments adjusted for embankment slope is width from edge of thoroughfare lane to hinge point Figure 9: Partially traversable cut slopes Policy Statement 2

11 For road sections with partially traversable cut slopes, the Clear Zone width should be calculated by using the formula given in Table 4. Table 4: Effective Clear Zone formula for partially traversable cut slopes Slope type Formula description Formulas Partially traversable cut slopes ECZ - Effective Clear Zone width S - is the embankment slope (Y/X) W 1 - is shoulder width ECZ = 1.2 x S 1 + W1 The Clear Zone calculation for partially traversable cut slopes is demonstrated in Example 8. Example 8 If CZ = 7.5m, W 1 = 1m, S = 1:1.5 and height = 2m ECZ = 1.2 x S 1 + W1, ECZ = 1.2 x (1/(1/1.5)) = 2.8m In the example above, the ECZ distance of 2.8 metres is located on a side of the slope. A pole may be installed at this location. However, it is preferable to place it on the flat surface at the top of the slope. Please note that for partially traversable slopes, the minimum width of the Clear Zone decreases as the steepness of the slope increases. For a vertical slope when S = 1:0, and ECZ = W 1. Part sentence missing here? Doesn t seem to make sense Safety Barriers and Non-transversable obstructions The Clear Zone widths calculated above do not apply when a pole is protected by a safety barrier. A safety barrier is designed to deform under impact. Therefore, it is important not to position poles within the barrier deflection area. In addition, it is necessary to allow sufficient room to perform pole maintenance, steeling and replacement. For these reasons when W-Barriers are used, poles should not be positioned any closer than one metre to the barrier face (see Figure 10). Other barriers, such as wire rope, deflect significantly more (i.e metres) and power poles must be installed outside the deflection distance. Policy Statement 3

12 Figure 10: Location of pole behind guard fence Where there is a non-traversable permanent obstruction in front of a proposed pole position, a rigid pole may be placed behind the obstruction, as close as installation and access requirements allow (see Figure 11). This applies when the obstruction is considered to be a bigger risk than the pole. Figure 11: Non-traversable permanent obstruction in front of pole Policy Statement 4