a spacing conveyor embodying features of the invention is shown in a conveyor system in fig1 . the conveyor system comprises a spacing conveyor 10 downstream of an infeed conveyor 12 . both conveyors advance in the conveying direction 14 , with the infeed conveyor feeding articles , such as packages 16 , single file onto the upstream end 18 of the spacing conveyor across a transfer plate 20 . the spacing conveyor transports the packages it receives toward its downstream end 19 . in a preferred embodiment , both the infeed conveyor and the spacing conveyor are realized as conveyor belts , such as modular plastic conveyor belts . the infeed belt 22 is shown as composed of a series of rows 24 of belt modules linked together by hinge pins 26 at hinge joints 28 between adjacent rows . the belt articulates at the hinge joints around drive sprockets 30 at its downstream end . the drive sprockets are mounted conventionally on a shaft ( not shown ) through central sprocket bores 34 . the drive shaft is coupled to a motor ( not shown ), which rotates the shafts and the sprocket and drives the belt . the infeed belt shown in fig1 is also characterized by article - supporting rollers 36 protruding past the outer surface 33 of the belt . but an infeed belt without rollers could be used as well . like the infeed belt , the spacing belt 38 is preferably a modular plastic conveyor belt with rollers 36 protruding past inner and outer surfaces 40 , 41 of the belt loop . the belt is trained between one or more drive sprockets 42 at the downstream end and one or more idle sprockets 43 at the upstream end . salient portions 44 of the rollers extend past the inner surface of the spacing belt into contact with bearing surfaces 46 underlying and supporting the belt on the rollers along an upper carryway portion 48 of the conveying path . as shown in fig2 , the bearing surfaces are positioned under the rollers along the carryway . the forward motion of the belt causes the rollers riding on the bearing surfaces to rotate in the direction of arrow 50 on axles 49 . packages 17 atop the rotating rollers are propelled in the conveying direction 14 by the rollers along the belt &# 39 ; s outer conveying surface 41 at a speed greater than the speed of the belt . the bearing surfaces may be realized as one or more continuous surfaces underlying each lane of rollers or as segmented surfaces that could be selectively raised and lowered as indicated by arrow 51 or otherwise moved into and out of contact with the belt rollers by hydraulic , pneumatic , or electromechanical devices , to accelerate packages or not , as required . the spacing belt also includes retractable stops 52 at spaced apart locations along the length of the belt . the stops in the example belt of fig1 and 2 are positioned on the hinge rod 26 between every third belt row . but the stops could be positioned between every row or spaced even farther apart than shown . the stops shown in the example have a plate - like portion 54 with a face 56 that receives a package propelled along the belt when the stop 52 is in a blocking position with its face above the roller top level 58 . the plate - like portions are connected to roller portions 60 at each end , as shown in the cut away sections along the carryway in fig1 . the roller portions reside in gaps 62 between adjacent belt rows . a bore 64 through the roller portions receives the hinge rod 26 , which serves as an axle on which the stop can rotate . actuators 66 positioned along the carryway at one or more positions along the length of the conveyor are selectively movable into and out of contact with the retractable stops , such as up and down as indicated by arrow 68 . the actuators are laterally aligned with the roller portions of the stops . the actuators , which may be moved by any conventional means , such as hydraulics , pneumatics , electromagnetics , mechanical systems , or air bursts , have an upper cam surface 70 with an entrance ramp 72 on which the stop &# 39 ; s roller portion 60 can ride as a cam follower . when in contact with the actuator &# 39 ; s cam surface , the stop rotates on the hinge rod into the blocking position . the stops are bistable with two stable positions : ( a ) the blocking position 52 ; and ( b ) a retracted position 52 ′. in the retracted position , the stop is below the roller top level 58 and cannot interfere with conveyed packages . because the stops are bistable , once an actuator engages a stop on the carryway , the stop remains in the blocking position until gravity causes it to return to its refracted position as the belt articulates about the idle sprocket 43 on its return to the carryway . thus , all the stops are automatically returned to their retracted positions just before entering the carryway . a sensor 74 , such an optical sensor , is shown in fig1 positioned at the downstream end of the infeed conveyor 12 . the sensor , which may include a light source emitting a light beam across the conveyor to a photodetector , is used to measure the length of each package . the passage of a package through the beam shadows the photodetector for a time proportional to the length of the package . as shown in fig3 , the sensor outputs a signal 75 on a signal line 76 indicative of the time the beam is shadowed , which is proportional to the length of a passing package . the signal is sent to a controller 78 , such as a programmable logic controller , a microcontroller , or other intelligent controller . with knowledge of the speed of the infeed conveyor , the controller can compute the length of the package by multiplying the speed by the time the beam was shadowed by the package . the controller then controls the activation of the one or more actuators 66 over control lines 68 to optimize the gap between consecutive packages . the operation of the spacing conveyor as controlled by the controller is illustrated in fig4 . in the exemplary operating mode depicted , the controller is programmed to maintain a selectable or predetermined nominal gap g of d ± d / 2 ( 0 . 5 d to 1 . 5 d ) between consecutive packages , where d is the distance between the positions of consecutive stops 52 . thus , in this example , the minimum allowed gap is 0 . 5 d . a first package p 1 whose length l 1 has been measured by the sensors is propelled atop the rollers on the spacing conveyor to a stop 52 a in the blocking position . because the length of the package is between 2d and 3d , i . e ., 2d & lt ; l 1 & lt ; 3d , the controller does not send actuation signals to the actuators to move the following two stops 52 b and 52 c to the blocking position . they remain in the retracted position below the roller top level and out of contact with the package p 1 . because l 1 ≲ 2 . 5 d , or , more generally , ( l 1 modulo d )≲ 0 . 5 d , the controller sends a signal to the actuator to move the next stop 52 d to the blocking position . this sets a gap between the tail end of the package p 1 and the head end of the trailing package p 2 as g 1 = 3d − l 1 ≳ 0 . 5d . in other words , the second blocking stop 52 d is the stop that trails the first blocking stop 52 a by the smallest distance greater than or equal to the sum of the length of the package and the minimum gap allowed , or ( l 1 + 0 . 5 d ) in this example . the package p 2 has a length l 2 as measured by the sensor and reported to the controller . after the trailing package p 2 is fed onto the spacing belt , it is accelerated forward to the stop 52 d already in the blocking position . because l 2 ≳ 2 . 5 d , or , more generally , ( l 2 modulo d )≳ 0 . 5 d , the controller sends a control signal to the actuator to move the second trailing stop 52 h to the blocking position . the stop 52 g between the tail end of the package p 2 and the next blocking stop 52 h is maintained in its retracted position to ensure that a gap of at least d / 2 is maintained . in this case , the gap g 2 between the tail end of the package p 2 and the front end of a trailing package is given by g 2 = 4d − l 2 ≳ 1 . 5d . in this way , the spacing conveyor maintains a gap of 0 . 5 d to 1 . 5d between consecutive packages . of course , it would be possible to program the controller to interpose greater gaps between consecutive packages . for example , where two lanes of packages are being conveyed on opposite sides of the spacing conveyor , either on a single belt with independently actuated stops or on each of a pair of side - by - side belts , it would be possible to stagger the packages from one lane to the other in anticipation of a clean merge downstream . in such a case , the gaps between consecutive stops in each package lane would have to be great enough to account for a staggered package in the other lane . and it is possible to achieve tighter controlled gaps by spacing the stops closer together along the length of the conveyor . although the spacing conveyor has been described in detail with reference to a preferred version , other versions are possible . for example , flat belts , slat conveyors , or other moving conveying surfaces with protruding rollers and retractable stops could be used instead of modular plastic conveyor belts . as another example , the stops do not have to rotate on hinge rods . they could alternatively pop up linearly when activated or be forced mechanically back to their retracted position . as yet another example , any sort of sensor that can measure the length of the packages can be used . and the actuators could be realized as rollers or as telescoping cam surfaces that may be extended along the length of the carryway to activate a selected number of consecutive stops . furthermore , the description was written as though the length of a package is its actual length when properly oriented . but , for packages oriented off - axis or for non - rectangular packages or articles , the length of the package would be the length of the longest line segment through the package parallel to the conveying direction , and not its actual length . so , as these few examples suggest , the spirit and scope of the claims are not meant to be limited to the preferred version described in detail .