Unsignalized Intersections

1. Unsignalized Intersections CTC-340

2. Hmwk • At end of powerpoint

3. STOP & YIELD controlled • Include TWSC, AWSC and Roundabouts • All models are based on a gap acceptance model

4. Gap Acceptance • Gap – distance between back of veh and front of next veh – not headway • Each gap can allow at least 1 veh to move • Vehicle using gap is based on a rank order • Figure next slide • Rank 1 – 2,3,5,6,15,16 • Rank 2 – 1,4,13,14,9,12 • Rank 3 - 8,11 • Rank 4 – 7,10 • Why this ranking?

5. Rank order

6. Conflicting Volume • Each movement must content with a different group of conflicting flows • Figure next slide • Look at footnotes • RT from major street do not conflict but some are counted in conflicting volume • 2 stage gap acceptance – median or TWLTL present – cars can cross 1 direction of traffic at a time

8. Conflicting Volume • Each movement must content with a different group of conflicting flows • Critical volume cmx = cpxPpvippj • cpx= potential movement capacity • pvi= probability that impeding veh movement j will not block flow (impedance factor) • ppj= probability that impeding ped movement j will not block flow (impedance factor)

9. Critical Gap • Minimum average acceptable gap that allows entry for 1 turning movement • Any gap smaller than critical gap is not used • Follow up time – minimum average acceptable time for a second queued vehicle to use a gap large enough to admit 2+ vehicles

10. Critical Gap • Critical Gap • tcx = tcb + tcHVPHV + tcGG – tcT – t3LT • Follow up time • tfx = tfb +tfHVPHV • tcb = base critical gap, T23.2 • tcHV= critical gap adjustment for HV • PHV= percent HV • tcG= critical gap adjustment for grade • tcT = critical gap adjustment for 2 stage gap acceptance • t3LT = critical gap adjustment for intersection geo • tfb = base follow up time, T 23.2

12. Potential Capacity • Assumes that all available gaps are used by subject movement • No higher priority movements will be at intersection • Assumes movement operates in exclusive lane • cpx = vcx[(e^-(vcx*tcx/3600))/(1-e^-(vcx*tfx/3600))] • vcx = conflicting flow for movement x

13. Impedance Effects • Effects due to higher ranked movements using a gap • Reduces the available gaps for the subject movement • Figure next page • First find movement capacity • cmx = cpxPpvippj • cmx = movement capacity • cpx = potential capacity • pvi = probability that movement i is not blocking subject flow • ppj = probability that pedestrian movement j is not blocking subject flow

14. Impedance Effects • Effects due to higher ranked movements using a gap • Reduces the available gaps for the subject movement

15. Impedance Effects • pvi = 1 – vi/cmi • vi = demand flow for impeding movement i • cmi = movement capacity for impeding movement i • The lower the v/c ratio for the impeding movement – the more likely that the subject flow will not be impeded • Rank 4 movements are impeded by many movements- may end up double counting impedance factor • p” = Pv1*Pv4*PvTH

16. Impedance Effects • p’ = 0.65p” – (p”/(p”+3) + 0.6SQRT(p”) • p’” = unadjusted impedance factor • p’ = adjusted impedance factor • Need to modify Major St LT when in shared lane • P*v1/4 = 1 – ((1-Pv1/4)/(1-(vmTH/smTH + vmRT/smRT))) • Ped impedance factor • ppj = 1 – (vj(w/Sp)/3600) • vj = ped flow rate • w = lane width • Sp= ped speed fps

17. Shared Lane Cap • Movement capacities assume exclusive lanes for each movement • When movements operate out of a shared lane • cSH = Svy/S(vy/cmy) • Capacity = total flow rate/ cSH

18. Upstream Signals • Gap acceptance assume random arrivals for all vehicles • If signalized intersections within ¼ mile – not true • Each platoon gives a different conflicting flow to the downstream intersection • Very complex

19. 2 stage gap acceptance • Occurs at divided highways or TWLTL • Increases capacity for minor street movements due to ability to cross 1 traffic stream at a time. • Limiting factors are the # of vehicles that can be in the median at the same time

21. Flared Lanes • Lane operates between exclusive lane and shared lane • Need to know average queue length of RT traffic • If max queue length <= # of flared spaces – operates like a separate lane • If max queue > # of flared spaces then capacity is a constrained

23. Delay • What is it • Control delay – includes time stopped in queue + time to decel + accel • Geometric delay – delay due to decel/accel to get thru intersection • HCM uses control delay as its MOE • dx = 3600/cmx +900T((vx/cmx-1)+SQRT((vx/cmx-1)^2 + (3600/cmx)(vx/cmx)/450T)) + 5

24. Delay • Delay is given for approach lane groups • Each exclusive lane or each shared lane • Major St LT • Major street thru assumed to have no delay • Depends on whether LT has an exclusive lane • Usually very small if it does occur

25. Queue Length • Q95x = 3600/cmx +900T((vx/cmx-1)+ SQRT((vx/cmx-1)^2 + (3600/cmx)(vx/cmx)/150T)) *(cmx/3600) • 95th percentile queue • Gives a sense of congestion at intersection • Higher queue means lower LOS

26. AWSC • Based on FIFO queue • Looks at probability of intersection in a certain condition • Determines the probability of each condition occurring given volumes and assesses the impact • Each approach affects the others • Iterative process

27. Roundabouts • Roundabouts must be Yield controlled and have a splitter island

28. Example 1 Spdlmt = 35 mph for A 45mph for B A 35’ 40’ B

29. Example 2

30. Homework

31. Prob 23-2 Determine the potential capacities for movements 1,7,8,9 • Prob 23-3 Determine the movement capacities for movements 1,7,8,9 • Prob 23-4 Determine the shared lane capacities for movements 7,8