Module 7 : Traffic Signal Design

Lecture 36 : Special Requirement in Traffic Signal

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	Effect of turning vehicles Right turning vehicles Right-turn signal phases facilitate right-turning traffic and may improve the safety of the intersection for right-turning vehicles. However, this is done at the expense of the amount of green time available for through traffic and will usually reduce the capacity of the intersection. Right-turn arrows also result in longer cycle lengths, which in turn have a detrimental effect by increasing stops and delays. While phases for protected right-turning vehicles are popular and commonly requested, other methods of handling right-turn conflicts also need to be considered. Potential solutions may include prohibiting right-turns and geometric improvements. The three criteria for right -turn phase is presented below: Traffic Volumes Delay: Separate right -turn phasing may be considered if the average delay for all right-turning vehicles on the approach is at least 35 seconds during that same peak hour. Collision Experience: Separate right -turn phasing may be considered if the critical number of reportable right -turn collisions has occurred. These are: (i) For one approach to the intersection, the critical number is five l right -turn collisions in one year, or seven in two years. (ii) For both approaches to an intersection, the critical number is seven right -turn collisions in one year, or eleven in two years. So the right turning vehicles affected saturation flow based on adjusted saturation headway. Finally actual values of right turning are calculated from right turn adjustment factor. The adjustments factor is calculated by following equations. Adjusted saturation headway, $$h_{adj} = h_{ideal}\times (P_{RT}\times e_{RT}+(1- P_{RT})\times 1)$$ Adjusted saturation flow, $$S_{adj} = \frac{3600}{h_{adj}}$$ Multiplicative right turn adjustment factor, $$f_{RT} = \frac{1}{1+P_{RT}(e_{RT}-1)}$$ $$S_{adj} = S_{ideal}\times f_{RT}$$ Numerical example If there is 15 percent right turning movement, eRT (through-car equivalent for permitted left turns) is 3, saturation headway is 2 sec; Find the value of Adjusted Saturation flow. Solution: Given $h_{ideal}= 2~sec$, $P_{RT} = 15\%(0.15)$, $S_{ideal}= 1800$, $e_{RT}=3$ Case 1: Find adjusted saturation headway as: $$h_{adj} = h_{ideal}\times(P_{RT}\times e_{RT}+(1- P_{RT})\times 1)$$ $$= 2 \times(0.15 \times 3+(1-0.15)\times 1)$$ $$= 2.6 sec/veh$$ Now, find adjusted saturation flow as: $S_{adj}=\frac{3600}{h_{adj}} = \frac{3600}{2.6} = 1385$. The adjusted saturation flow is 1385 vph. Case 2 Find the adjustment factor to calculate adjusted saturation flow based on ideal saturation flow (1800) $$f_{RT} = \frac{1}{1+P_{RT}(e_{RT}-1)}$$ $$= \frac{1}{1+0.15(3-1)}=0.77$$ $$s_{adj} = S_{ideal}\times f_{RT} = 1800\times 0.77 = 1386$$ The adjusted saturation flow is 1386 vph. The result is same from both cases. Left turning vehicles Lft turn adjustment factor for saturation flow rate is as follows: For exclusive lane $f_{LT}$ is 0.85 and for shared lane $f_{LT}=1.0-0.15~P_{LT}$, where pLT is the proportions of left turns in lane group. Normally in left turn, separate signal phase are not provided at intersection as per Indian standard.