This degree project was carried out at the transport design office Setec International, in Paris, from August 3rd to December 18th, 2020. I was part of the team in charge of the post-evaluation of the A150 highway.Permanent observatories are organizations that bring together public and private actors in order to collect early and systematically data to analyze the effects of a transport infrastructure. They have been set up since the 1980s, mainly on French highways. In particular, they contribute to the production of a posteriori assessments known as "LOTI". The objective of this project is to qualitatively evaluate the contribution of these observatories to socio-economic evaluations, both for "classic" and "wider economic" effects. The methods used are the "practical" experience of carrying out the LOTI assessment of the A150, the study of reports describing the results of the A39 and A71 observatories, and the analysis of reports from the State services on the quality of LOTI assessments.The permanent observatories limit some of the pitfalls of ex-post assessments linked to the measurement of classic effects, especially traffic and journey times. They also highlight the effects of a construction site and the strategies of the economic actors. However, they retain certain limitations of ex-post assessments such as the spatial and temporal scale, or the concessionaire's willingness not to disclose certain data. Thanks to early and regular data collection, permanent observatories seem to contribute significantly to the analysis of classic and wider economic effects, they limit the risk of poor quality analyses. Nevertheless, they do not seem indispensable for the production of studies of satisfactory quality.The rare data available on the cost of the observatories suggest that it is very low compared to the amounts invested in construction. Thus, in comparison with these costs, this end-of-study project gives a positive qualitative assessment of the contribution of permanent highway observatories to socio-economic studies. However, this study is limited by the amount of data available due to the non-mandatory nature of setting up permanent observatories.