As urban roads become more complicated and older, subsurface subsidence often occurs in underground utilities and spaces. The city of Seoul has been engaged in activities to prevent these problems since 2014. As one of the preventive activities, subsurface cavities that may cause road cave-ins have been mainly searched for using ground-penetrating radar (GPR) surveys. GPR has sensors for nondestructive survey. The purpose of this study was to analyze cavity signal patterns and compare them with cavities found by multichannel GPR equipment. Also, an optimal signal analysis method was proposed for multichannel GPR data. GPR tests were conducted on 204 road cavity test sections, and GPR signal patterns were analyzed to classify the signal shape, amplitude, and phase change. Four types of multichannel GPR equipment were used to detect subsurface cavities in a pilot road section in Seoul. Various types of filters were applied to time domain data to examine the optimal signal processing. GPR signals on cavity sections were mostly symmetric (or symmetric in some cases) hyperbola shapes in the longitudinal or transverse direction. The amplitude of GPR signals reflected from cavities was stronger than that from other media. No particular pattern of the amplitude was found because of non-uniform media and nearby utilities. In many cases where cavities reached the bottom of an asphalt concrete layer, the signal phase was reversed. However, no reversed signal was found in subbase, subgrade, or deeper locations. The time domain analysis of the raw data showed that the four types of GPR equipment produced reverse and strong signal reflection due to the low dielectric permittivity of air in the cavity compared with that in neighbor materials. Also, an asymmetric parabolic curve was commonly observed. An optimal signal-processing method for detecting road cavities was determined: zero-setting and background removal should be applied for all types of GPR equipment, and bandpass filtering can be optionally applied to remove high-frequency noise or direct waves. All types of evaluated multichannel GPR equipment were found to be suitable for detecting road cavities located at 1.0 and 1.5 m depths after a suitable filtering process. In general, GPR signals on road cavity sections had a symmetric hyperbola shape, relatively strong amplitude, and a reversed phase. However, because of the uncertainties of underground materials, utilities, and road cavities, GPR signal interpretation is still difficult. To perform quantitative analysis for road cavity detection, more GPR tests and signal pattern analysis need to be conducted.