In many developing countries, especially Nigeria, informal sector activities such as roadside trading, hawking as well as roadside squatting has become increasingly prevalent. Although these activities are often seen as survival strategies for the low-income population with no other means of earning a living, nonetheless, they have negative impacts on the environment and traffic flow and can also compromise the safety and security of the traders. This study aims to explore the factors that contribute to the growth of this phenomenon on encroached highways, urban roads, and open spaces, and to examine its effects on both the environment and the traders. Using a narrative review approach, the research analysed relevant books, journals, conference papers, reports, and policy documents extracted from Scopus, Web of Science and Google Scholars databases. The study findings revealed that economic crises and high levels of unemployment in developing countries have led to an increase in street trading and roadside vending. In addition, the lack of effective urban planning and enforcement of development control laws has also played a significant role in the proliferation of these activities on encroached urban roads and green spaces. Consequently, on highways, the absence of organized and standardized Rest and Service Areas (RSA) at regular intervals in developing countries, especially Nigeria, has facilitated the emergence of roadside markets. Furthermore, this situation is compounded by the location of agrarian settlements along the highways. Thus, to address these issues, the research recommends that the government reorganize, integrate, and upscale the activities of street traders, while also improving urban planning and enforcing road right-of-way barriers to protect urban green spaces and conclusively, the introduction of standard RSA at regular intervals along highways would provide motorists with necessary rest and services, discouraging them from stopping indiscriminately on transits, which often leads to the emergence of roadside markets.