The Transportation Department has proposed rules to improve the safety of tank trucks that carry hazardous materials by trying to reduce the danger of deadly fires and poisonous leaks in highway accidents.

The regulations would require thousands of tank trucks to be fitted with stronger manhole cover assemblies and improved pressure relief valves designed to prevent chemicals from leaking.

Under the proposal, the carriers also would face stricter maintenance standards and more frequent inspections.

The government would allow the carriers five years to complete the retrofitting job.

Studies have shown that manhole cover assemblies and pressure relief valves often fail when a cargo tank overturns, resulting in leakage and fires.

"Some relief valves permit the release of hazardous liquids in an overturn and many cargo tanks have four or more such valves," the DOT said in a statement.

"The proposal clarifies the existing requirement that such valves may not permit this sort of leakage in an overturn," the department said. "The proposal also notes that a large percentage of pressure relief valves failed basic operating tests."

The rules require that all affected cargo tanks undergo a formal maintenance program and major repairs would have to be performed by a qualified shop.

All tanker trucks would be inspected annually instead of every two years, as now required.

Manufacturers of new cargo tanks would have to hold a certificate and each cargo tank would need to have an inspector's approval.

The types of tanks affected include those that transport gasoline, fuel oil and other flammable and combustible liquids. Also included are tank cars that are used to transport ammonia, liquefied petroleum gas, flammable liquids, poisons and corrosives.