HIGHWAY ACCIDENT REPORT
Adopted: October 17, 1973
MULTIPLE-VEHICLE COLLISION
FOLLOWED BY PROPYLENE CARGO-TANK EXPLOSION
NEW JERSEY TURNPIKE, EXIT 8
SEPTEMBER 21, 1972 
NTSB Number: HAR-73/04
NTIS Number: PB-225032/AS
SYNOPSIS
At 8:25 p.m., on September 21, 1972, a tractor-semitrailer (tank) carrying propylene liquid petroleum gas sideswiped a Greyhound bus (carrying no passengers) in the southbound lanes of the New Jersey Turnpike about one mile north of Exit 8. After impact, the bus, while rotating clockwise and sliding across the highway, was struck by a southbound automobile. The tractor-semitrailer scraped, then straddled the turnpike's median guardrail, jackknifed, spun into the northbound lanes, and overturned. Before overturning, the tractor-semitrailer was struck by a northbound automobile.

Fire, which had erupted at the tractor as it scraped the median guardrail, spread to propylene which was leaking from the cargo tank's damaged plumbing. After the fire had burned for about 25 minutes, the cargo tank exploded in a ball of flame; segments of the tank rocketed more than 1,300 feet northeast and 500 feet southwest of the tractor-semitrailer.
As a result of the accident, the driver of the tractor-semitrailer suffered severe burns and multiple fractures; the busdriver received minor injuries. The two occupants of the northbound automobile which struck the semitrailer were killed. Twenty-eight persons, including seven police officers, were injured -- none seriously -- by the explosion of the cargo tank.

The National Transportation Safety Board determines that the probable cause of the initial collision was the evasive steering and skidding of the bus into the path of the overtaking tractor-semitrailer. Override of the median guardrail by and subsequent overturn of the tractor and the semitrailer were caused by the inability of the median guardrail to resist the forces generated by the tractor-semitrailer.

The initial fire was caused by friction sparks when the tractor-semitrailer scraped the median guardrail, which ignited fuel escaping from the tractor's damaged left-side fuel tank. Secondary fire was propagated by propylene which escaped from a rupture(s) in the cargo tank's external pipes. Contributing to the escape of propylene were (1) the exposed position of the cargo tank's external pipes, (2) the inadequacy of the "plumbing guard" to protect the pipes from impact damage, and (3) the failure of the flow-cutoff system to function as intended by applicable Federal regulations.

Explosion of the cargo tank was caused by extended exposure of a local segment of the tank shell to direct flame and by resultant overheating of that portion of the tank shell, which weakened it below design strength and permitted a break in the tank body. Contributing to the weakening of tank metal was the absence of tank-overheating countermeasures by emergency crews whose arrival was delayed by traffic congestion on a limited-access highway. The number of injuries was increased by lack of understanding of the range of the hazard.

RECOMMENDATIONS

The National Transportation Safety Board recommends that:

1. The Bureau of Motor Carrier Safety of the Federal Highway Administration OWA) study the existing regulations and requirements in 49 CFR 178.337, regarding LPG cargo tanks, for the purpose of instituting more explicit rulemaking toward reducing the likelihood of leakage and subsequent catastrophic failure of such cargo tanks in a variety of foreseeable types of accident crashes. (Recommendation No. H-73-37)

2. The National Highway Traffic Safety Administration, in cooperation with the Bureau of Motor Carrier Safety and the International Association of Chiefs of Police, Inc., as an addition to the official manual which supplements the Highway Safety Program Standard relating to police traffic services, develop criteria and procedures for the demarkation and evacuation of danger zones at hazardous-materials accidents occurring on highways. (Recommendation No. H-73-38)

3. The Bureau of Motor Carrier Safety and the National LP-Gas Association adopt a tentative distance of 1,400 feet as a danger-zone radius for M-331 cargo tanks carrying liquefied petroleum gas which are exposed directly to fire. Such a radius should be used pending study and additional experience on which to base a more definitive danger-zone radius. (Recommendation No. H-73-39)

4. The Bureau of Motor Carrier Safety, the National LP-Gas Association, and the National Fire Protection Association jointly consider formally recognizing the fact that, while an LPG cargo tank continues to be directly exposed to fire, the opening and subsequent closing of a safety-relief valve (or valves) signals the dropping of liquid level below the fire-application point, and serves as an indicator of impending failure of the tank from heat softening. This information could then be included in approved safety manuals relating to LPG cargo-tank fires. (Recommendation No. H-73-40)