Unsignalized Intersections CTC-340

Hmwk At end of powerpoint

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

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?

Rank order

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

Conflicting Volume –Each movement must content with a different group of conflicting flows –Critical volume c mx = c px  p vi p pj –c px = potential movement capacity –p vi = probability that impeding veh movement j will not block flow (impedance factor) –p pj = probability that impeding ped movement j will not block flow (impedance factor)

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

Critical Gap –Critical Gap t cx = t cb + t cHV P HV + t cG G – t cT – t 3LT –Follow up time t fx = t fb +t fHV P HV t cb = base critical gap, T23.2 t cHV = critical gap adjustment for HV P HV = percent HV t cG = critical gap adjustment for grade t cT = critical gap adjustment for 2 stage gap acceptance t 3LT = critical gap adjustment for intersection geo t fb = base follow up time, T 23.2

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 c px = v cx [(e^-(v cx *t cx /3600))/(1-e^-(v cx *t fx /3600))] v cx = conflicting flow for movement x

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 c mx = c px  p vi p pj c mx = movement capacity c px = potential capacity p vi = probability that movement i is not blocking subject flow p pj = probability that pedestrian movement j is not blocking subject flow

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

Impedance Effects p vi = 1 – v i /c mi v i = demand flow for impeding movement i c mi = 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” = P v1 *P v4 *P vTH

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-P v1/4 )/(1-(v mTH /s mTH + v mRT /s mRT ))) –Ped impedance factor p pj = 1 – (v j (w/S p )/3600) v j = ped flow rate w = lane width S p = ped speed fps

Shared Lane Cap Movement capacities assume exclusive lanes for each movement –When movements operate out of a shared lane c SH =  v y /  v y /c my ) Capacity = total flow rate/ c SH

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

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

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

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 –d x = 3600/c mx +900T((v x /c mx -1)+SQRT((v x /c mx - 1)^2 + (3600/c mx )(v x /c mx )/450T)) + 5

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

Queue Length –Q 95x = 3600/c mx +900T((v x /c mx -1)+ SQRT((v x /c mx -1)^2 + (3600/c mx )(v x /c mx )/150T)) *(c mx /3600) –95 th percentile queue –Gives a sense of congestion at intersection –Higher queue means lower LOS

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

Roundabouts –Roundabouts must be Yield controlled and have a splitter island

Example 1 A B 35’ 40’ Spd lmt = 35 mph for A 45mph for B

Example 2

Homework

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