Unit 7 Speed, Travel Time and Delay Studies

Transcription

1 Unit 7 Speed, Travel Time and Delay Studies

2 Introduction Speed, travel time and delay are all related measures that are common used as indicators of performance for traffic facilities Measure of Effectiveness (M.O.E.) Average travel speed Arterials, two-lane rural highways Control delay Signalized and STOP-controlled intersections Speed, Density Freeway A basic understanding of the quality of operations on a facility, and in ways that directly relate to defined performance criteria 2

3 Introduction Speed an important factor in evaluating high accident locations as well as in the other safety-related investigations speed measurements are most often taken at a point (or a short section) of roadway under conditions of free flow spot speed studies used to determine general speed trends, to help determine reasonable speed limits, and to assess safety speed (SMS, ) is inversely related to travel time u s 3

4 Travel time Introduction must be measured over a distance, most travel time measurements are made over a significant length of a facility travel time studies are generally done during times of congestion specifically to measure or quantify the extent and causes of congestion 4

5 Introduction Delay delay is a portion of total travel time delay along an arterial might include stopped time due to signals, mid-block obstructions, or other causes of congestion the most frequently used forms of intersection delay include: Stopped time delay Approach delay Time-in-queue delay Control delay 5

6 Introduction Travel-time delay is the difference between the actual travel time traversing a section of highway and the driver s expected or desired travel time is more of a philosophic approach, as there are no clearly accurate methodologies for determining the expected travel time of a motorist over a given section of highway 6

7 Spot Speed Studies Spot speed defined as the average speed of vehicles passing a point on a highway, are measured at a given spot or location, the result is a distribution of speed time mean speed (TMS, u t ) traffic engineer is interested in conducting spot speed studies under conditions of free flow 7

8 Spot Speed Studies Speed definitions of interest Average or time mean speed Standard deviation of speeds 85th percentile speed Median Pace a 10-mph increment in speeds that encompass the highest proportion of observed speeds 8

9 Spot Speed Studies Uses of spot speed data Establishing the effectiveness of new or existing speed limits or enforcement practices Determining appropriate speed limits for application Establishing speed trends at the local, state, and national level to assess the effectiveness of national policy on speed limits and enforcement 9

10 Spot Speed Studies Specific design applications determining appropriate sight distances, relationships between speed and highway alignment, and speed performance with respect to steepness and length of grade Specific control applications for the timing of yellow and all red intervals for traffic signals, proper placement of signs, and development of appropriate signal progressions Investigation of high-accident locations at which speed is suspected to be a contributing cause to the accident experience 10

11 Spot Speed Studies Measurement techniques Measurement of travel times as vehicles traverse a short measured distance along the highway Use of hand-held or fixed-mounted radar meters Other advanced techniques Detectors Video technologies 11

12 Spot Speed Studies Some practical measurement issues Concealment issue - All field personnel need to make an effort to conceal themselves and there activity Sampling issue - Under the best of circumstances, it is still virtually impossible to record the speed of every vehicle passing a study site Even in light traffic, vehicle platoons may form 12

13 Spot Speed Studies Proper location for speed studies measurements should include drivers freely selecting their speeds, unaffected by traffic congestion spot speed studies are rarely made under conditions of heavy, or even moderate traffic 13

14 Travel-time Studies Travel-time studies are often coordinated with delay observations at point of congestion along the study route Travel-time information is used for many purposes, including: To identify problem locations on facilities by virtue of travel times and/or delay To measure arterial level of service, based on average travel speed and travel times To provide necessary input to traffic assignment models, which focus on link travel time as a key determinant of route selection To provide travel-time data for economic evaluation of transportation improvements To develop time contour maps and other depictions of traffic congestion in an area or region 14

15 Travel-time Studies Field Study Techniques Floating-car technique the test-car driver is asked to pass as many vehicles as pass the test-car in this way, the vehicle s relative position in the traffic stream remains unchanged the test-car approximates the behavior of an average vehicle in the traffic stream result in estimates of the average travel time through the section is generally applied only on two-lane highways 15

16 Travel-time Studies Maximum-car technique the driver is asked to drive as fast as is safely practical in the traffic stream without ever exceeding the design of the facility the measured travel time represent the lower range of the distribution of travel times, which are more indicative of a 15th percentile than an average Average-car technique the driver is instructed to drive at the approximate average speed of the traffic stream result in estimates of the average travel time through the section 16

17 Travel-time Studies the number of test-car runs from a low of 6 to 10 to a high of 50, depending upon the type of facility and the amount of traffic another technique may be used to collect travel times Roadside observers record license plate numbers as vehicles pass designated points along the route. The time of passage is noted along with the license plate number. License plate numbers matching The use of videotaping on elevated vantage points Tracing and recording every vehicle s travel time 17

18 Travel time contour 18

19 A plot of elapsed time versus distance Stopped delay 19

20 Intersection Delay Studies Stopped delay the primary delay measure at intersection before 1977 Total control delay MOE for signalized and STOP-controlled intersections Time-in-queue delay plus time loss due to deceleration from and acceleration to ambient speed The study methodology recommended in the HCM 2000 is based on direct observation of vehicle-in-queue at frequent intervals and requires a minimum of two observers 20

21 Average travel speeds plotted versus segments of a route 21

22 Intersection Delay Studies The following should be noted: The method is intended for under-saturated flow conditions, and for cases where the maximum queue is about 20 to 25 vehicles The method does not directly measure accelerationdeceleration delay but uses an adjustment factor to estimate this component The method also uses an adjustment to correct for errors that are likely to occur in the sampling process Observers must make an estimate of free-flow speed before beginning a detail survey. 22

23 Intersection Delay Studies Actual measurements start at the beginning of the red phase of the subject lane group There should be no overflow queue from the previous green phase when measurement start The following tasks are performed by the two observers : Observer 1 Keeps track of the end of standing queues for each cycle by observing the last vehicle in each lane that stops due to the signal. This count includes vehicles that arrive on green but stop or approach within one car length of queued vehicles that have not yet started to move. 23

24 Intersection Delay Studies At intervals between 10s to 20s, the number of vehicles in queue are recorded on the field sheet. The regular intervals for these observations should be an integral divisor of the cycle length. Vehicles in queue are those that are included in the queue of stopping vehicles (as defined above) and have not yet exited the intersection. For through vehicles, exiting the intersection occurs when the rear wheels cross the STOP line; for turning vehicles, exiting occurs when the vehicle clears the opposing vehicular or pedestrian flow to which it must yield and begins to accelerate. At the end of the survey period, vehicle-in-queue counts continue until all vehicles that entered the queue during the survey period have exited the intersection. 24

25 Intersection Delay Studies Observer 2 where : T During the entire study period, separate counts are maintained of vehicles arriving during the survey period and of vehicles that stop one or more times during the survey period. Stopping vehicles are counted only once, regardless of how many times they stop. Q T I Q s Viq = I s * V *0.9 T = average time - in - queue, s/veh = time interval between time - in - queue counts, s V iq = sum of all vehicle - in - queue count VT = total number of vehicles arriving during the study period,vehs 0. 9 = empirical adjustment factor 25

26 Intersection Delay Studies Adjustment for acceleration/deceleration delay requires that two values be computed: The average number of vehicles stopping per lane, per cycle, and The proportion of vehicles arriving that actually stop where : V V N N c L V SLC STOP SLC VSTOP = N N c L = number of vehicles stopping per lane, per = total count of stopping vehicles,vehs = number of cycles included in the survey = number of lanes in the survey lane group cycle ( veh/ln/cycle) 26

27 Intersection Delay Studies where : where : FVS = V V STOP T FVS = fraction of vehicles stopping d = T Q + ( FVS CF ) d = total control delay, s/veh CF = correction factor 27