Traffc Sgnal Tmng: Basc Prncples 2 types of sgnals Pre-tmed Traffc actuated Objectves of sgnal tmng Reduce average delay of all vehcles Reduce probablty of accdents by mnmzng possble conflct ponts Objectves may conflct! Development of a Traffc Sgnal Phasng and Tmng Plan Select Sgnal Phasng Determne f protected or permtted left turns wll be used HCM Gudelnes - Consder usng protected phase when the product of left turnng vehcles and opposng traffc volume exceeds: 50,000 durng the peak hour for one opposng lane 90,000 for two opposng lanes 0,000 for three or more opposng lanes Two Phase and Three Phase Sgnal Operaton F07_07
Typcal Phasng Confguratons and Sequencng F07_08 Example: Determnng Sgnal Phasng Plan F07_09 Example: Recommended Sgnal Phasng F07_0 2
Development of a Traffc Sgnal Phasng and Tmng Plan Establsh Analyss Lane Groups Determne crtcal lane groups Calculate cycle length Allocate green tme Typcal Lane Groupngs for Analyss F07_ Summary There s one lane (or lane group) for each phase requrng the maxmum amount of effectve green tme. For ths lane or lane group, we have the crtcal lane volume (CLV). There s an effectve green tme requrement and crtcal lane volume for each phase n the cycle. The "requred green" for the cycle s the sum of the effectve green requrements for each phase. We must provde at least ths amount of effectve green (per hour) to pass the traffc (wthout queung). 3
Determnng Cycle Length Typcally wll mnmze delay to stopped vehcles (thus mnmze C). C = n = L CLV s *Ths equaton assumes Xc =. (Xc s the crtcal v/c raton for the ntersecton. If Xc =, ths mples ntersecton operates at full capacty. Occasonal cycle falures could occur usng ths approach, because of the randomness of vehcle arrvals. Determnng Cycle Length From Mannerng text: C mn L Xc = n CLV X c = s *Round C to nearest 5 seconds. Choose Xc based on desred degree of utlzaton of the ntersecton. Cycle lengths should be n the range of 40 20 seconds (unless ntersecton s very complex (5+ phases). Determne Splts Determne how long each phase wll receve rght-of-way ( G+ A) = n = CLV s CLV s ( C L) + l *Equaton n your book s for Effectve Green Tme (secton7.5.6) L = total lost tme for Cycle 4
Real World Constrants Cycle length constrants: We would lke to mplement cycle lengths n the range of 40-20 seconds. Cycle lengths of less than 40 seconds waste too much tme (lost tme), and for cycles much over 20 seconds, motorsts sometmes thnk that the "lght" s malfunctonng, and enter the ntersecton on red. Dsplay tme constrants: We don't show the drver such thngs as two second greens ("Show the drver thngs he's seen before.") Some traffc engneers mght use dfferent values, but n ths class, we wll use mnmum (G+A) values of: 2 sec (exclusve left turns) 5 sec (through) Peakng: We tme traffc sgnals for the peak 5 mnute flow rate, just lke we made our calculatons on for capacty and level of servce for unnterrupted flow. Real World Constrants Composton: As a rough approxmaton, assume that each truck s the equvalent of 2.0 passenger cars (ths value s compatble wth the Hghway Capacty Manual (HCM)). The HCM uses ths to adjust saturaton flow rate from ts "deal" value of 900 pcphgpl (passenger cars per hour of green per lane). As an approxmaton, we wll use: s = 900 f hv where s = saturaton flow rate (vphgpl) Where f hv s the proporton of trucks n the stream. Snce we are takng E T =2, ths reduces to: f hv = + PT Our approxmaton to saturaton flow rate, then, s: s = 900 + P T (Ths s both a slght varaton and smplfcaton of the HCM technque for sgnalzed ntersectons, but t s suffcent for our purposes). Real World Constrants Unprotected left turns: Unprotected left turns are those who turn left on the "green ball." We call those turnng left on a green arrow "protected" left turns. As an approxmaton, assume that each unprotected left turn s the equvalent of 2.0 through vehcles. Pedestran constrants (where pedestran volumes are sgnfcant): Ped tme = 5 sec + walk tme (Walk rate s about 4 ft/sec) All sgnal tmng methods are approxmate - checkng and adjustments must be made n the feld. 5
Example Fnd cycle length and splts for the ntersecton confguraton shown below. Assume saturaton headways of 2. sec/veh-lane and lost tmes of 5 sec/phase for all approaches. Example 2 Part A As cty traffc engneer of Attapulgus, Georga, you are responsble for tmng the town's traffc sgnal, whch operates wth two phases. Lost tme s 4.5 seconds for each of the two phases and peak hour factor s 0.83. Peak hour data for each of the four approaches s gven n the table below. Peak Hour Percent Percent APPROACH Volume Trucks Left Turns EB 548 5 6 WB 672 7 7 NB 598 4 8 SB 606 2 6 Intersecton geometry s as shown below. Fnd the requred cycle length and splts. Example 2 Part B The mayor s up for re-electon and has promsed, f returned to offce, to provde funds to sgnfcantly mprove these two streets. What cycle length and splts would you mplement f the ntersecton was mproved by addng lanes as shown below? 6
Example Part C Well, the mayor's opponent, who campagned on a platform of fscal conservatsm, won the electon. Ths means that there wll be no major mprovements to the ntersecton. However, the new mayor s wllng to foot the bll for a can of pant, and you do have enough pavement wdth to add left-turn bays for the east-west approaches. For your "new" ntersecton (shown below), can you re-tme the sgnal to gve a more reasonable operaton than what you got n part a? 7