E470 building 10 miles of median cable barrier
By Peter Samuel
At E470 they are going heavily for tensioned cable median barrier. 16.5km (10 1/4 miles) of three cable barrier are being installed in the first major contract following a safety study presented to directors in July 2006. Tensioned cable barrier on sacrificial steel posts seems to be a valuable new safety feature so long as there is at least 7m or 8m (23ft to 26ft) median space for the designed sideways deflection of an errant vehicle hit.
The cables seem to be safer than either solid concrete barrier or steel W-beam guardrail. Most tollroads have been building solid concrete barrier in recent years in place of W-beam steel guardrail because the solid barrier prevents a much higher proportion of errant vehicles getting through into the opposite direction of traffic. However against solid concrete vehicles bounce back into their own roadway with little driver control and suffer considerable damage from the hits.
The steel posts of the new tensioned cable systems are deliberately weakened at ground level so they break away or bend flat on impact allowing the tensioned cables to control the errant vehicle.
Cables seem to catch a decent proportion of errant vehicles without nearly as much impact trauma. And like steel guardrail they are considerably cheaper to install than concrete - about $60/m (under $20/ft) vs $250/m ($75/ft) for concrete. Maintenance costs of the more sacrificial cable and guardrail systems are greater than concrete.
A safety report to E470 Public Highway Authority by Jake Kononov of DiExSys, safety engineering consultants of Littleton CO said: "Recent studies and field experience suggest that cable rail has significantly lower injury rates compared to steel or concrete barrier."
Between 2001 and 2005 E470 had 17 cross median incidents defined as where a vehicle crossed the median and went into the opposing roadway. Several were fatalities but because of light traffic others involved no collision. As traffic increases the dangers increase, and the toll authority plans to put cable in all 76km (47mi) of their road within a few years.
The E470 grassy central median is mostly 12.8m (42ft) wide so barrier is not regarded as mandatory under conventional thinking - which in the US says barrier is mandatory only for medians of less than 10m (33ft) and in Europe 9m (30ft). Beyond 15m (49ft) of grassy median width barrier has not normally been considered helpful.
E470's median is right in the middle of the range conventionally regarded as "barrier optional" according to AASHTO advice. But some DOTs are now declaring it policy to install cable barrier in all median down to 10m (33ft) if there is any history of cross median accidents, and in highways with more than 20k daily traffic.
Cable barrier works better than solid barrier on slopes and is generally more forgiving because of it applies its restraining force more gently with its stretching and sideways deflection. That is an advantage so long as there is a median space of about 7.6m (25ft) because in anything narrower the deflection sideways will get an errant vehicle dangerously close to the opposite direction of traffic.
If E470 were widened inwards by a lane each direction the median would be reduced to 5.5m (18ft) and a solid concrete barrier would be needed.
Disadvantages of barrier are cited as:
- reduced clear zone for recovery by an errant vehicle
- an increase in the number of reported crashes (but great reduction in severity)
- limiting the options for maintenance and emergency vehicles to do U-turns
- some reduction in snow storage capacity
- costs of installation and maintenance
Ohio, North Carolina and Washington state have done considerable amounts of cable barrier in the last few years. But TxDOT has perhaps more experience with cable barrier than any other US state. They recommend solid barrier on medians of less than 7.6m (25ft). They tension the cable to a maximum deflection of 2.4m (8ft) and recommend a minimum clear distance of 3.7m (12ft) between the travel lane edge and the cable. Posts are spaced 3m to 5m (10ft to 17ft) apart. Costs in Texas run between $9 and $12/foot ($30/m and $40/m).
The E470 installation is Cass TL4 - a 1.03m (40in) high system from Trinity Highway Products in Dallas TX. See http://www.highwayguardrail.com/products/cassTL4.html
There are several other suppliers of similar tensioned cable barrier. It is sometimes installed at a lower height 0.75m (29") but the higher installation works better against trucks and SUVs.