Mast arm traffic signals and cantilever highway signs have an inverted L-shaped structure and this geometry results in high loading demand to their support foundations when subjected to high wind loading. This MS thesis is motivated by an NCDOT research need statement related to design and construction challenges inherent in inverted L-shaped traffic signal structures located in coastal environments. For these coastal structures wind loading demand can be very high associated to design wind speeds in the order of 209 KMH (130 MPH) or higher. These high design wind speeds result in high loading demand to the foundation system particularly for traffic signal structures with long mast arms lengths (e.g., lengths of 25 m or greater), and the high surface areas exposed to wind in highway signal structures with areas of 20 m² or greater. Exacerbating the design challenge, these coastal projects often involve a small right-of-way resulting in a small footprint for the foundation system and often involve sites with poor geotechnical conditions such a loose sand deposits with a high-water table. The first part of this thesis includes a state of practice (SOP) and literature review study performed as part of the NCDOT project. The SOP component of the study involved a survey questionnaire with follow-up phone interviews to 12 coastal U.S departments of transportation to document the foundation systems commonly used for supporting coastal mast arm traffic signal structures, as well as typical dimensions, and design procedures. The SOP also involved review and comparison of current design procedures used by the participating DOTs. The SOP study found that the most commonly used foundation system to support these structures are single drilled shafts with diameter typically ranging from 0.6 to 1.5 m (2 and 5 ft). Additional to the SOP study the NCDOT project involved a literature review related to foundation systems under high loading demand of inverted L-shaped structures that involve a complex load combination that includes lateral loading, lateral bending moment, and torsion. The literature review identified a few studies that investigated the effect of combined lateral loading and torsion. For example, Hu (2003) and Thiyyakkandi (2016) reported significant capacity reduction in the lateral resistance of single drilled shafts when subjected to coupled torsion loading. This topic warrants additional research. This SOP study shows that coastal DOTs have a challenging task to design safe and cost-effective foundations systems for coastal mast arm traffic signals due to the loading demand associated to the high winds and long mast arm lengths, and the poor geotechnical conditions often encountered in coastal environments. The selection of the commonly used drilled shaft option often results in large diameters (up to 1.5 m) and long embedment depth (up to 11 m).The second part of this MSCE thesis reports on three failure case studies of inverted L-shaped highway sign structures in Puerto Rico affected by the 2017 Hurricane Maria. For each case history the details of the cantilever signal structures are provided and the geotechnical conditions as well as the failure mechanism is described. All three cantilevered highway signal structures rotated significantly about their zenith as result of high wind pressures applied to the structure. The load capacity of the drilled shafts for each case history are predicted using different design methodologies reported in the SOP study. The analyses performed for the three failure case histories revealed that the ultimate load method based on Broms (1964 a and b) predicts lateral loading short-pile type for all 3 cases. This suggests this method is conservative as only one of the three case histories showed some lateral rotation and the main failure mode was torsional rotation. The Broms method, as originally published (Broms 1964), is considered conservative also due to the neglect of lateral resistance of the upper 1.5 diameters of the drilled shaft for clayey foundation soils. The use of p-y models to analyze the lateral load and bending moment loading condition was found to be more realistic as it did not predict failure under this type of loading which is consistent with the field observations for these three foundation failure case histories. The torsion capacity predictions, using different methods, were all lower than the estimated torsion loading demand experienced by the different drilled shafts, thus predicting correctly the observed main mode of failure for these three foundation failure case histories.