# The Effects of Signal System and Traffic Flow on the Sound Level
## 1 Introduction  
In recent decades, many researchers have investigated various environmental noise research topics such as railway noise [1; 2], aircraft noise [3; 4], wind turbine noise [5; 6], marine/port noise [7; 8; 9], noise and health [10; 11], and, noise measurement method [12; 13]. Hong Kong is an economic city with probably the highest traffic intensity and density on roads in the world. Thus, traffic noise is also one of the important research topics (e.g., [14; 15; 16; 17; 18; 19; 20]). At the end of 2015, there were more than 720,000 licensed vehicles [21] running on the road network where the total road length was around 2100 km [22]. Because of the spatial constraints, traffic noise is a critical problem in Hong Kong. The current study is a combination of the two research topics (i.e., road traffic noise and traffic signal system). There are very few studies about that. [PERSON] et al. [23] and [PERSON] et al. [24] conducted simulations to study the effects of traffic signal coordination/traffic light control on noise. In their studies, there were no on-site measurement results. Most research works about road traffic noise only focused on mathematical modelling, evaluation method, road gradient and vehicle type effects amongst others. For example, [PERSON] et al. [25] developed a new mathematical model which was used for estimating the road traffic noise levels at sites along a freeway where the traffic volume fluctuated from a maximum of 900 vehicles per hour in the daytime to a minimum of 300 vehicles per hour at night. [PERSON] et al. [26] proposed a statistical evaluation method for traffic noise under the existence of background noise. The effectiveness of the proposed method was experimentally confirmed by applying it to the traffic noise data measured in a complex sound environment. [PERSON] et al. [27] studied the influence of different acoustical characteristics of urban road traffic including powered-two-wheelers on noise annoyance. [PERSON] and [PERSON] [28] carried a set of site measurements on the kerbs of 19 inclined roads and studied the effect of road gradients on the traffic noise. [PERSON] and [PERSON] [29] proposed a highway traffic noise prediction model to account for the interaction of car vehicles on asphalt and concrete road surfaces. [PERSON] and [PERSON] [30] determined theexposure of the urban population to road traffic noise living in residential complexes built at different time periods between 1950 and 2000 in Hong Kong. Moreover, most research works about a signal-controlled system only focused on intersection modelling, signal optimization, and travel time minimization etc. There were no considerations about noise level. For example, [PERSON] and [PERSON] [31] developed the continuum signalized intersection model to accurately account for three realistic complications, namely signal offsets, queue spillbacks, and complex signal phasing schemes. [PERSON] et al. [32] addressed the problem of simultaneous route guidance and traffic signal optimization problem, where each vehicle in a traffic network was guided on a path and the traffic signals serving these vehicles were set to minimize their travel times. The aforementioned studies mainly focused on either traffic noise or traffic signals. The present study mainly focused on the effect of a traffic signal on the roadside sound levels emitted by controlled vehicles. To the best knowledge of the authors, the present research is the first one which contains a set of on-site measurements. Several sets of roadside sound levels were measured for different scenarios, such as \"stop and go\" (i.e., showing red and green signals) and \"non-stop\" (i.e., showing full green signals). The terms \"stop and go\" and \"non-stop\" can be regarded as a traffic road with and without traffic signals, respectively. A measurement site with minimum influence due to other vehicles and noisy construction works was selected. A set of microphones were used on each side of the road in each sound measurement.