Explosives, like all hazardous materials (hazmat), are subject to regulations to ensure safe handling and transportation, among other things. Hazmat regulations are coordinated with international standards and generally govern the labeling, packaging, and transportation of hazmat in commerce. Explosives are one of nine classes of hazmat. In order to be transported, explosives must be assigned a classification. The classification, which includes a number that denotes the risk level of the explosive (from most to least hazardous), dictates associated transportation requirements, such as by which transportation modes the explosives can travel and how they are packaged. For example, class 1.1 explosives, which pose a mass explosion hazard, cannot travel by aircraft but can travel by truck. Meanwhile, certain class 1.4 explosives, which pose a minor explosion hazard, that meet specific requirements can travel by aircraft or U.S. Postal Service. Classifications also include “compatibility groups” that denote which explosives can be transported together. For example, the regulations do not allow blasting detonators, which are used to trigger an explosive device, to be transported in the same truck as primary explosive substances. Unlike some other classes of hazmat that can be self-classified (meaning the shipper classifies the material), in order to be classified, explosives must be first examined by one of six PHMSA-approved third party test labs. The explosives manufacturer, which ultimately submits an application to PHMSA for classification, first selects and hires one of the test labs and makes a sample of the explosive available to the test lab for examination. The test lab uses international standards to test the material and to recommend a classification. PHMSA then reviews the manufacturer’s application, including the test lab’s report and recommended classification, and approves a classification for the explosive (see fig. 1). In 2010 and 2014, the DOT OIG reported weaknesses in PHMSA’s management and oversight of the approvals process. For example, in 2010, the DOT OIG raised questions about the effectiveness of PHMSA’s oversight and found that PHMSA had not inspected any of the test labs in 10 years and that test labs did not always submit annual reports as required. In 2014, the DOT OIG reported that PHMSA had addressed these issues. However, the 2014 report found PHMSA evaluation forms missing for many explosives classification reviews, a situation that DOT OIG noted was an internal control weakness. As a result, the DOT OIG recommended PHMSA require the use of evaluation forms to document its review of explosive classification applications. According to a DOT OIG official, PHMSA effectively addressed this recommendation in 2015. PHMSA’s oversight of the classification of new explosives includes two key parts—(1) approving and monitoring the test labs and (2) reviewing and approving the manufacturers’ applications for classification of a new explosive and the labs’ test classification recommendations. PHMSA has several activities to approve and oversee test labs, but its efforts to ensure consistency are limited by PHMSA’s lack of a systematic approach to developing and issuing guidance for these labs. In response to the DOT OIG’s findings of oversight weaknesses from 2010, PHMSA strengthened its activities to approve and monitor test labs. Specifically, to ensure that test lab examiners meet the requirements specified in regulation, PHMSA established an approval process that includes interviewing examiners and reviewing test lab and examiner qualifications and recertifying test labs every 5 years through an on-site inspection. PHMSA officials stated that in addition to ensuring compliance with regulations, an important goal of PHMSA’s oversight of test labs is to promote consistency across test labs. PHMSA officials stated that efforts to promote consistency are particularly important given turnover—half of test labs were approved in 2012 or later. However, while PHMSA works to promote consistency through various types of communications with test labs, it lacks a systematic approach to determining what guidance is needed and to issuing such guidance. Internal control standards state that agencies should communicate quality information so that external parties can help the agency achieve its objectives and address related risks. Likewise, we have previously reported that agencies benefit from procedures that continually reassess and improve guidance processes and documents to respond to the concerns of regulated entities. According to PHMSA officials, regulations and international standards described in a United Nations test manual set forth certain requirements for testing new explosives. However, PHMSA officials told us they give test labs flexibility in how to apply these requirements on a case-by-case basis when testing new explosives to recommend classifications. PHMSA officials told us that granting this flexibility allows test labs to use their expertise and professional judgment. For example, PHMSA officials noted examiners can deviate from the proscribed tests in the test manual if they justify their reasoning for doing so based on their expertise, and officials at one test lab stated that, given the variation in the specific attributes of different new explosives, such flexibility can improve their ability to effectively test a new explosive. However, PHMSA and some test lab officials also stated that this flexible approach can lead to inconsistencies across test labs, such as a test lab examiner being unaware of a common deviation from the proscribed test or similar types of explosives being subject to different tests. For example, one examiner reported learning of an “unwritten rule” that test labs can use alternative tests in cases where multiple samples of the explosive cannot be destroyed due to costs, such as for large and expensive explosives. Four of the six test labs we spoke to said written guidance from PHMSA could help address “unwritten rules” such as commonly used modifications to the test manual. Although four of the five manufacturers we spoke to said the labs are consistent in quality of testing, two noted that test lab report quality can vary. PHMSA has ongoing efforts to promote consistency across test labs. Specifically, PHMSA officials stated that they: Discuss issues, best practices, and PHMSA’s recommended approaches with test lab examiners during in-person annual meetings and quarterly teleconferences. PHMSA also distributes agendas and minutes associated with these teleconferences and meetings. Five of the six test labs we spoke to said PHMSA’s quarterly teleconferences and annual meetings are helpful to share issues and good practices. However, three test labs noted that test lab examiners are hesitant to volunteer information or ask questions during these meetings since the test labs compete with one another for business. As one test lab noted, if one test lab has information that the others do not, this test lab has an incentive not to mention this information in order to have a business advantage over competitors. Issue letters of interpretation to communicate PHMSA’s views on specific issues in response to questions or concerns, which are posted on PHMSA’s website. However, PHMSA officials and one test lab stated that while the letters of interpretation include topical guidance, they are hard to find, are not organized by topic, and can contradict each other. As described above, PHMSA’s approach to providing guidance is not systematic, and therefore PHMSA may be missing areas where more guidance would be beneficial to helping test labs understand PHMSA’s expectations and to improving PHMSA’s ability to reach its objective of consistency among test labs. Moreover, PHMSA officials acknowledge that they currently do not have a comprehensive written document that encompasses all PHMSA guidance for test labs. They stated that they are currently evaluating whether to issue such a document. PHMSA officials stated that the development of such a document could involve compiling the content of existing written communication such as the letters of interpretation, which, as described above, PHMSA officials stated are not currently easily accessible. However, PHMSA officials have not specified a systematic approach for these efforts, including an effort to identify test labs’ needs such as explaining the “unwritten rules” that affect PHMSA’s expectations for test labs. Without such an approach to improving its guidance, PHMSA may not be providing test labs with the information needed to effectively meet PHMSA’s goal of promoting consistency. PHMSA has a multi-part application review and approval process for the classification of new explosives, including a check for completeness by a program officer, two levels of technical review, and the completion of an evaluation form (see fig. 2). According to PHMSA officials, PHMSA’s role in approving classifications, which is outlined in regulation, is essential to fulfilling its role in regulating hazmat transportation in the U.S. Furthermore, PHMSA officials say its application review process serves quality assurance purposes, since test labs, which are paid by the manufacturers, compete for business and may be pressured by manufacturers to provide a specific classification. Two of the six test labs we spoke to reported facing pressure from manufacturers on how to conduct tests or which classification to recommend. A third test lab also stated that manufacturers often aim to have their explosive classified as class 1.4 since, as mentioned previously, such explosives can be transported by aircraft. One manufacturer noted that having an explosive classified as a 1.4 to travel by aircraft makes the product more competitive since air travel is the fastest option to get the product to an overseas customer. PHMSA officials stated that they are checking for multiple things in their technical reviews, and that in addition to ensuring that explosives are classified correctly, these technical reviews also help PHMSA to improve test labs’ performance. Specifically, PHMSA officials stated that they keep notes in an Excel spreadsheet on recurring issues identified in application reviews, which are then used to inform topics for annual and quarterly meetings with the test labs.