# Speed-Flow-Geometric Relationship for Urban Roads Network
## 5 Discussion  
The majority of the previous models of the speed-flow-geometric relationship used few geometric parameters and suffered from various limitations. To surmount these problems, we used a wide range of geometric and traffic features and studied their impacts on the speed-flow-geometric relationship models using a multilinear regression analysis. This allowed us to identify the combined influence of median, the number of lanes, side friction condition, driveways density, intersection density, traffic calming speed device density, and right-turn driveway density on the main relationship between traffic speed and traffic volume on urban roads. The paired \(t\)-test showed that road which has no median with one lane, regardless of the condition of side friction, requiring the conversion of traffic volume into pcu/h before the use of the developed models. The remaining three categories, namely no median, two lanes, and high side friction; with median, two lanes, and high side friction; and no median, one number of lanes, and high side friction did not provide significant difference when they converted the traffic volume from veh/h to pcu/h. Thus, these three models could be applied using traffic volume as a veh/h only. According to the various circumstances of cross-sectional features--namely median, number of lanes, and side friction condition--a series of trend lines represented the results of the developed speed-flow relationship. All these figures of plotted lines offered as the tools for the application of the developed models in the field by the authority of urban roads. In general, the developed speed-flow-geometric relationship models can be applied for five road's categories of cross-sectional parameters (median, number of lanes, and side friction condition). In practice, the developed model for each road's category can be applied to any urban road that corresponds to this cross-section category, regardless of the types of longitude parameters contained in this segment. Alternately, any developed model for each category can be used broadly regardless of the type, density of any longitudinal parameter (LP) on this urban road segment, unless certain constraints for a specific parameter limits the application of this model. In detail, if an urban road without median has only one lane and low side friction, the speed-flow-geometric relationship model of this category can be applied for any kind of longitudinal parameter (LP) values except if the access driveways exceed 100 per 1 km. This value reduced to 40 the access driveway per 1 km if the urban road comes with a median and high side friction. Regarding the traffic calming speed device (TCSD), it is applicable for all five of the road's categories with two constraints. These two constraints are the maximum number of traffic calming speed devices (TCSD) for the developed models for the urban road segment without median and the high side friction conditions are 10/km and 50/km, respectively, for one and two lanes. Except for the instance of an urban road with no median, one lane, and high side friction, all of the model's equations may be applied to any degree of density for the longitude parameter (LP) of intersection density. For the urban road with no median,one lane and high side friction, the speed-flow-geometric relationship may be limited to eight intersection density per km. Otherwise, all these previous certain restrictions of longitude parameters (LP)-values, the developed speed-flow-geometric relationship models of this study could be used by the urban road's authority as a significant design and planning tool. The average travel speed (ATS) values of the vehicles on 1 km of the urban roads was reduced considerably with the increase of density as one per 1 km in traffic calming speed devices, access driveway, and intersection by 1.28 km/h, 0.52 km/h, and 1.26 km/h respectively. This reduction of the average travel speed (ATS) relied on the various combination of median, number of lanes, and side friction conditions.