5Summary – design exception opportunities#
Table 5 – Design exception opportunities and mitigating measures
| Design Parameter | Design Exception Opportunity | Mitigating measures |
|---|---|---|
| Cross section (Section 4.1) | Two lane two way roads | • Reduced lane widths and edge clearance - potential cross section reduction of 0.2 m to 0.6 m. | • Signs: − narrow shoulders − speed detection and display. • Road lighting • Driver behaviour surveillance • Speed enforcement • Site surveillance and incident recording |
| Two lane one way carriageway | • Narrow right lane with larger vehicles restricted to the left lane – potential cross section reduction of 0.45 m to 0.75 m for each carriageway | • In addition to the measures above, regulatory signage to restrict larger vehicles to using the left lane only | |
| Sight distance (Section 4.2) | • Combined use of: − Deceleration parameter D = 0.46 − Reduced driver reaction time of 1.5 s − Increased object height = 0.8/1.25 m • Resultant reduced sight distance requirement of approximately 24% | • Speed enforcement/ compliance • Lighting • Variable speed limits based on weather conditions • Ongoing incident monitoring and debris removal • Variable message signs • High skid resistance pavement • Removal of anti-gawk screens on the insides of curves |
| Coordination of horizontal and vertical alignment (Section 4.3) | • Aesthetic considerations for horizontal and vertical alignment may not need to be observed | • Speed enforcement / compliance • Lighting • Alignment markers • Arrow boards or variable message signs • Delineation devices such as bollards and temporary road safety barriers • Ongoing monitoring |
| Horizontal alignment (Section 4.4) | • Compound curves, broken back curves and reverse curves may be acceptable • Absolute maximum side friction values for cars not meeting heavy vehicle requirements • Adverse crossfall may be acceptable | • Speed enforcement / compliance • Reduced speed limit for larger vehicles • Additional warning signage to alert heavy vehicle drivers to the appropriate warning speed |
| Vertical alignment (Section 4.5) | • Grades greater than maximums for short lengths within road works. • Development of sag curves, on the basis of sight distance criteria without meeting comfort criteria | Nil identified |
| Acceleration / deceleration lanes (Section 4.6) | • Deceleration lanes – allow increased speed reduction prior to entry of deceleration lane • Acceleration lanes: − Assessment of vehicle speeds based on empty loads − Allow lower entry speed based on lower entry volumes compared with normal entry ramps | • Speed enforcement / compliance • Risk based approach considering number of site vehicles and volume of through traffic • Timing of site vehicle movements in traffic gaps introduced by traffic control features • Ability to assist site vehicles with shadow and tail vehicles • Risk based approach considering actual vehicle loads • Active warning signs |
| Road Safety Barriers (Section 4.7) | • Risk based approach to remove barrier requirements • Application of permanent road risk considerations for temporary barriers | • Risk based approach − Duration of works − Length of exposure to traffic − Depth of excavation − Angle of approach − Lighting of alignment • Other protection methods (eg road plates after hours) • Increased delineation devices • Lighting |