A nondescript truck with three large tanks bolted to its bed rumbles down a residential street in Silicon Valley. No one looks twice. It is a common sight. Plastered on the truck's side are several diamond-shaped, black-and-white placards bearing four-digit numbers. The placards mean that the cargo the truck carries is hazardous. The black-and-white markings warn that the hazard is a highly poisonous substance. The four-digit number will tell policemen or firemen responding to a spill or leak what the chemical is and how to deal with it - if they have the requisite 500-page federal guidebook.
This truck is part of the widespread commerce of hazardous substances, which is being subjected to fresh scrutiny since the leak of a highly toxic gas at the Union Carbide pesticide plant in Bhopal, India, earlier this month.
There are no reliable statistics on the extent of the trafficking in hazardous cargo in the United States. Typical estimates range from 100,000 to 250,000 shipments daily. Ballpark government figures put the total tonnage of dangerous substances shipped at 4 billion tons annually. This includes a wide range of materials - everything from nail polish to poisonous chemicals to plutonium. Another commonly cited figure is that 1 in every 10 trucks on the road carries hazardous materials. But no one really knows for sure, says Lee Santman, head of the Department of Transportation's (DOT) Hazardous Materials Transportation Bureau.
While estimates of the quantities involved are less than precise, it is clear that the volumes involved are tremendous. And there is agreement that it is the transportation of such hazardous materials, more than their use in the factory, which represents the greatest threat of a US catastrophe on the order of Bhopal.
In transportation ''there are simply more uncontrollable variables,'' observes Edith Page, an analyst at the Congressional Office of Technology Assessment (OTA), who is directing a recently initiated study on the subject.
''The transport of toxic chemicals poses a definite risk, but we have invested a lot of time and energy keeping this risk minimal,'' says Stuart Blanton, manager of hazardous materials transportation for E. I. DuPont de Nemours Inc., a company with a strong reputation for safety.
According to Mr. Blanton, DuPont has slashed its transportation accident rate by 80 percent in the last six years. While he declines to provide statistics confirming his assertion, Mr. Santman says DOT's figures reflect substantial progress since 1978-79, when there were several major rail and truck accidents involving hazardous materials.
DOT says the number of incidents involving hazardous materials dropped from 9 ,000 in 1981 to 5,700 last year. Some of this is due to changes in the reporting requirements that eliminate minor mishaps like paint and battery-acid spills. Still, the number of deaths reported dropped from 27 to 8 over the same period, and the number of injuries declined from 640 to 190.
''These are awfully small numbers, so it is difficult to interpret changes. But there does seems to be a definite trend there, although that is no reason for complacency,'' Santman says.
Concern over dangerous cargo dates back to the late 1800s. Originally, it was dealt with contractually between shippers and carriers. In the early 1900s, the federal government assumed responsibility. Since then the system has evolved gradually, until today there are some 3,500 commodities that the government classifies as hazardous. There are also 250 to 300 substances (like nitroglycerin) that DOT deems too dangerous and prohibits shipment of altogether.
''This is not some crash program put in place 10 or 15 years ago in the crucible of high public attention. The system has grown steadily and quietly,'' says Santman, who attributes much of its success to this gradual growth.
Another reason for the low accident rates has been the efforts of responsible companies like DuPont. The industry, through the Chemical Manufacturers Association, sponsors a group called Chemtrek. It was set up to respond to accidents involving hazardous chemicals all around the country. Its ''800'' number is plastered on trucks and bills of lading for many hazardous shipments. Chemtrek experts give advice on how to handle an spill or leak and, if the situation warrants, assemble an on-site team to advise local authorities on the proper procedures.
''We get about 3,000 calls per month. Two (hundred) to 400 represent actual emergencies, but less than 20 are really serious,'' says Chemtrek director Joe Mayhew.
Yet these very volunteer efforts represent an aspect of the situation that concerns many people: The chemical industry has largely kept information and expertise on the hazardous chemicals they produce behind their chain-link fences.
''Although the industry says it is willing to provide training and information, it only provides what it is comfortable in providing, rather than what local communities would like,'' says OTA's Ms. Page.
The industry closely guards information about its shipments, Mr. Blanton acknowledges, because this knowledge ''can give away our marketing strategy.''
Many communities discover that shipments of dangerous chemicals are passing through their towns only after an incident occurs for which they were ill-prepared. A rising chorus of demands for more information resulting from incidents of this sort was a major factor behind the commissioning of the current OTA study.
''I think there are sufficient laws on the books, but there is a major problem with inspection. For every responsible company like DuPont, there is some sleazy operation out there cutting corners,'' says Julie Jordan, a transportation analyst with the National Conference of State Legislatures.
The inspection problem is greatest with trucking, which accounted for all the deaths and 65 percent of the injuries in interstate transportation of hazardous materials in the last three years, according to the DOT. Santman admits this is the weakest point: ''(There are) a lot of trucks out there, and our 130 inspectors are simply not enough.''
Trucks do not carry the large quantities that trains or ships haul, but they travel closer to where people live. And in a number of ways, they have proven more difficult to regulate.
A problem peculiar to trucking is that of driver qualification. In some states all a person needs to drive a rig hauling hazardous materials is an ordinary driver's license. In another, getting the chauffeur's license required involves correctly answering an additional 10 questions on the written test. In other states, however, drivers must meet significant additional requirements before they are licensed to drive hazardous cargo.
There is also a significant problem with drivers operating on multiple licenses from different states. Several months ago a truck hauling a load of Navy torpedoes overturned while speeding around a cloverleaf on a downtown freeway interchange in Denver. Authorities discovered that the driver had several licenses, including one that was suspended because he had been convicted of drunken driving.
The DOT requires special training for drivers carrying a few specific types of cargo, including radioactive material. But, in general, the agency prefers to leave operator licensing to the states.
For some time now states have been showing a new interest in policing hazardous shipments within their boundaries. California, for instance, has assigned about 120 members of the highway patrol to the regulation of hazardous materials shipments. Strict legislation has been passed in Illinois, Tennessee, and Georgia following major incidents. Although more than 40 states have now adopted federal standards, ''there is still a great deal of variation in the level of effort,'' Ms. Jordan says.