WO2019013694A1

WO2019013694A1 - A method and a system for adjusting vehicle speed

Info

Publication number: WO2019013694A1
Application number: PCT/SE2018/050754
Authority: WO
Inventors: Björn Johansson; Henrik SVÄRD; Oscar Flärdh; Christoffer NORÉN
Original assignee: Scania Cv Ab
Priority date: 2017-07-13
Filing date: 2018-07-10
Publication date: 2019-01-17

Abstract

A method and a system for adjusting vehicle speed of a motor vehicle(1), the method comprising: -retrieving driver acceptable constraints defining allowed automatic vehicle speed variations, -in response to a detection of at least one nearby vehicle (3, 4), determining a current gap to the nearby vehicle and obtaining an expected speed trajectory of the nearby vehicle, -based on the current gap, the expected speed trajectory of the nearby vehicle, and at least one minimum gap size (d1, d2), determining traffic acceptable constraints set to ensure that a future gap to the nearby vehicle will correspond to at least the minimum gap size, -based on a current vehicle speed and the driver and traffic 1 acceptable constraints, determining an intended speed trajectory of the motor vehicle satisfying at least one of said constraints, -controlling the vehicle speed based on the intended speed trajectory.

Description

A method and a system for adjusting vehicle speed

TECHN ICAL FI ELD OF THE I NVENTI ON The present invention relates to a method for adjusting vehicle speed of a motor vehicle accord ing to claim 1 and to a system for adjusting vehicle speed according to claim 14. The invention further relates to a computer program, a computer program product, and a motor veh icle. By a motor vehicle is here intended a vehicle which is powered by an internal combustion engine and/or by an electric motor. In particular, but not exclusively, the method is intended for use in a heavy motor vehicle such as a truck or a bus. By a gap is herein intended a gap between the present vehicle and a nearby vehicle in terms of either distance or time.

BACKGROUND AND PRIOR ART The cost of fuel for motor vehicles, e.g . cars, trucks and buses, represents a significant expense for the owner or user of the vehicle. A wide variety of different systems have therefore been developed for reducing fuel consumption , e.g . fuel-efficient engines and fuel-economising cruise controls. Such fuel- economising cruise controls aim to reduce fuel consumption by adjusting the driving to the characteristics of the road ahead , so that unnecessary braking and/or fuel-consuming acceleration may be avoided . For example, by taking topographic information about the road section ahead of the vehicle into account, the speed may be temporarily increased before e.g . an uph ill slope, so that downshifting to a lower transmission mode can be avoided or delayed . I n this way, a total energy consumption can be reduced . Also information about road curvature and legal speed limits along the road section ahead of the vehicle can be taken into account.

Modern motor vehicles can furthermore be equ ipped with radar technology to measure a distance to a lead veh icle travelling ahead of the motor vehicle. Some motor veh icles can also be equipped with a control system to automatically maintain a specified gap to the lead veh icle, as long as the speed of the motor vehicle does not exceed a set speed , such as a legal speed limit. Such a control system is usually referred to as an Adaptive Cruise Control (ACC), a Radar Cru ise Control, or an Autonomous Cru ise Control system.

Driving behind a lead vehicle may save fuel due to reduced air resistance, but may also result in that normal fuel saving systems, such as certain fuel-economising cruise controls, cannot be fu lly utilised due to the risk of coming too close to the lead vehicle, regardless of whether the motor vehicle is driven with an activated ACC system or not. Certain fuel saving systems and functions are therefore deactivated when driving behind a lead vehicle.

US20140244129 discloses a method for controlling vehicle speed in a motor vehicle having a cru ise control. A speed setpoint used by the cruise control is adapted in order to achieve fuel efficiency, using offsets to take e.g . topography into account. According to the method , the traffic density in the vicin ity of the vehicle is determined , and the offsets to the speed setpoint are reduced in proportion to the traffic density. In this way, heavy traffic in the vicinity of the vehicle will lead to a reduced possibility of adjusting the speed setpoint to account for a varying road gradient.

SUMMARY OF THE I NVENTION

It would be desirable to provide a method and a system that offers an improved possibility to adjust the vehicle speed of a motor vehicle in the vicinity of surrounding traffic, so that the motor vehicle can be fuel efficiently operated whenever the traffic situation allows.

To better address this concern , a method and a system as defined in the independent claims are provided . According to one aspect, a method for adjusting veh icle speed of a motor vehicle in which the vehicle speed is controlled using a cruise control is provided . The method comprises:

- retrieving driver acceptable constraints defining how the vehicle speed is allowed to be automatically varied by means of the cruise control,

- in response to a detection of at least one nearby vehicle travelling ahead of and/or behind the motor vehicle, determining a current gap to the at least one nearby vehicle and obtaining an expected speed trajectory of the at least one nearby vehicle,

- based on the determined current gap, the expected speed trajectory of the at least one nearby vehicle, and at least one predefined minimum gap size, determining traffic acceptable constraints set so as to ensure that a future gap to the at least one nearby vehicle will correspond to at least the minimum gap size,

- based on a current vehicle speed , the driver acceptable constraints and the traffic acceptable constraints, determining an intended speed trajectory of the motor veh icle satisfying at least one of said constraints,

- controlling a powertrain of the motor vehicle such that the vehicle speed is adjusted based on the intended speed trajectory.

According to another aspect, a system for adjusting veh icle speed of a motor vehicle in which the vehicle speed is controlled using a cruise control is provided . The system comprises a control un it configured to:

- retrieve driver acceptable constraints defining how the vehicle speed is allowed to be automatically varied by means of the cruise control,

- in response to a detection of at least one nearby vehicle travelling ahead of and/or behind the motor vehicle, determine a current gap to the at least one nearby vehicle and obtain an expected speed trajectory of the at least one nearby vehicle,

- based on the determined current gap, the expected speed trajectory of the at least one nearby vehicle, and at least one predefined minimum gap size, determine traffic acceptable constraints set so as to ensure that a future gap to the at least one nearby vehicle will correspond to at least the minimum gap size,

- based on a current vehicle speed , the driver acceptable constraints and the traffic acceptable constraints, determine an intended speed trajectory of the motor vehicle satisfying at least one of said constraints,

- control a powertrain of the motor vehicle such that the vehicle speed is adjusted based on the intended speed trajectory.

The proposed method and system provide a means of, in the presence of nearby veh icles, adjusting a speed setpoint (also referred to as reference speed) used by the cruise control such that the motor vehicle is fuel efficiently operated taking road gradients into account, while still ensuring that the motor vehicle does not come too close to the nearby vehicle(s) as a result of the adjustment of the speed setpoint. Since expected speed trajectories of nearby vehicles are taken into account when setting the traffic acceptable constraints, the traffic acceptable constraints can be set based on an expected future development of the traffic situation , not merely on a present traffic density. The adaptation to nearby traffic can thereby be improved . Even in heavy traffic, small departures from the set speed of the cruise control may be permitted in order to e.g . improve fuel efficiency, if th is can be achieved without violating the traffic acceptable constraints. Thus, the method and system according to the invention improve the possibility to operate the motor vehicle in accordance with the driver's intentions also in the presence of surrounding vehicles.

The method and system may be adapted to simu ltaneously take both a trailing vehicle travelling behind the motor vehicle and a lead vehicle travelling ahead of the motor vehicle into account. Different minimum gap sizes may be defined for trailing vehicles and for lead vehicles. The expected speed trajectory of the at least one nearby vehicle may be estimated on-board the motor vehicle based on predefined assumptions, but it may also be estimated or otherwise determined elsewhere and communicated to the motor vehicle, such as from the nearby vehicle or from a central node.

The driver acceptable constraints may define how the vehicle speed may be varied by the cruise control in dependence on road gradient and road curvature in the absence of nearby vehicles. These constraints may typically be set based on input from a driver of the motor vehicle, including e.g . a desired set speed of the cruise control, a maximum and a minimum allowed deviation from the set speed and a selected performance mode, such as an economy mode for min imising fuel consumption , or a power mode for maximising power output. In accordance with the driver acceptable constraints, the actual speed setpoint that the cruise control uses to regulate the vehicle speed may depart from the set speed by an amount that depends on the selected performance mode. The driver acceptable constraints may also be set based on input relating to e.g . speed limits along the road . For example, speed limits may be communicated to a communication unit in the motor veh icle from a central communication node, or included in map data. The set speed of the cruise control may be used as an input parameter when determining the intended speed trajectory, if not included in the driver acceptable constraints.

The traffic acceptable constraints may be expressed in terms of distance to be covered by the motor veh icle as a function of time, i.e. by specifying a minimum distance and a maximum distance that may be covered, taking the minimum gap size(s) into account.

Preferably, the intended speed trajectory of the motor veh icle is set so that it satisfies all of said driver acceptable constraints and traffic acceptable constraints. Alternatively, if all constraints cannot be fulfilled , preference may be given to some of the constraints. The system according to the invention may either form part of a cruise control, or be a separate system which may be used in combination with a cruise control.

Preferred embodiments of the invention are disclosed in the dependent claims.

Accord ing to one embodiment, if the at least one nearby vehicle includes a lead vehicle travelling ahead of the motor vehicle, the traffic acceptable constraints include a hard constraint on a minimum forward gap to the lead vehicle, which hard constraint must be satisfied by the intended speed trajectory. The method may e.g . include evaluating whether said hard constraint is satisfied by the intended speed trajectory, and , based on the evaluation , set a status of the intended speed trajectory as allowed or non-allowed . In an optimization based method , the hard constraint may be given as an in put to a solver, which produces a speed trajectory that satisfies the hard constraint. I n this embodiment, an intended speed trajectory that does not satisfy the hard constraint on the minimum forward gap cannot be set. It is thereby ensured that the motor veh icle cannot come closer to the lead vehicle than the minimum forward gap.

Accord ing to one embodiment, if the at least one nearby vehicle includes a trailing veh icle travelling beh ind the motor vehicle, the traffic acceptable constraints include a soft constraint on a minimum rearward gap to the trailing vehicle, which soft constraint is allowed to be selectively satisfied by the intended speed trajectory. Th is allows the motor veh icle to keep travelling at e.g . set speed if the trailing vehicle travels much faster than the present vehicle, such as in a situation in which a driver of the trailing vehicle intends to overtake the motor vehicle. In this case, the gap may be allowed to fall below the minimum allowable gap instead of increasing the speed of the motor vehicle to prevent the gap from becoming too small.

Accord ing to one embodiment, the driver acceptable constraints include a maximum allowable distance deviation from a desired constant set speed trajectory of the motor vehicle. The constant set speed trajectory is the trajectory that the motor veh icle would follow at a constant speed equaling the set speed of the cruise control. The maximum allowable distance deviation may preferably be topography dependent, i.e. determined in dependence on the topography ahead of the motor vehicle.

Accord ing to one embodiment, the driver acceptable constraints include a position dependent maximum and/or minimum allowable vehicle speed . In other words, a maximum and/or minimum allowable veh icle speed as a function of vehicle position is defined . Preferably, road gradient along the road ahead of the motor vehicle is taken into account. If set as a hard constraint, the maximum and/or minimum allowable vehicle speed may not be exceeded . Accord ing to one embodiment, the determination of the intended speed trajectory comprises a rule based method and/or an optimization based method . A rule based method is easy to implement and also to understand , maintain and troubleshoot. A rule based method also produces a predictable resu lt. An optimization based method may produce a less pred ictable result, but the result is on the other hand typically better than that from a rule based method . For example, if optimized with respect to fuel efficiency, the optimization based method would typically result in a more fuel efficient intended speed trajectory. It is also possible to use a combination of rule based and optimization based methods, such as by generating a first set of possible speed trajectories using a rule based method , and a second set of possible speed trajectories using an optimization based method , and based on some criteria selecting the intended speed trajectory from said sets. Of course, both rule based and optimization based methods may include simu lations.

Accord ing to one embodiment, the determination of the intended speed trajectory includes:

- calculating a number of possible speed trajectories according to predefined rules,

- based on said constraints, setting a status of the possible speed trajectories to allowable or non-allowable, - based on predefined selection criteria, selecting one of the allowable speed trajectories as the intended speed trajectory.

As explained above, it is of course also possible to calculate a number of possible speed trajectories using optimization .

According to one embodiment, the predefined selection criteria are set so as to select the most fuel efficient allowable speed trajectory.

Accord ing to one embodiment, the method further comprises determining at least one intended control trajectory associated with the intended speed trajectory, wherein the at least one intended control trajectory is used to control the powertrain of the motor vehicle. The at least one control trajectory may include a torque trajectory, includ ing driving and braking torques to be demanded from the powertrain , and/or a gear selection trajectory defin ing which gear(s) to be selected . The at least one control trajectory can be used to control the powertrain such that the vehicle speed is adjusted based on the intended speed trajectory. The intended speed trajectory and the associated at least one control trajectory are preferably determined simultaneously.

Accord ing to one embodiment, determining a current gap to the nearby vehicle comprises measuring the current gap. Preferably, measuring the current gap can be realised using a radar equ ipment. The motor vehicle may for th is purpose be equ ipped with forward and rearward radar equ ipment configured to sense lead vehicles and trailing vehicles, respectively, and to measure a gap to said vehicles. The radar equ ipment may also be used to detect a nearby vehicle and to calcu late its speed . An alternative to using radar equ ipment is to use optical cameras or light detection and rang ing (LI DAR). Accord ing to one embodiment, the at least one minimum gap size is determined based on at least one of a vehicle speed , a vehicle acceleration , brake action , tyre quality, road conditions, weather, and traffic cond itions. In th is case, the vehicle speed/acceleration may be an absolute vehicle speed/acceleration of the motor vehicle or a relative speed/acceleration in relation to a nearby vehicle.

The invention also relates to a computer program comprising instructions which , when the computer program is executed by a computer, cause the computer to carry out the proposed method and to a computer-readable medium comprising instructions which , when executed by a computer, cause the computer to carry out the proposed method . It will be appreciated that all the embodiments described with reference to the method aspect of the present invention are applicable also for the system aspect of the present invention . That is, the system may be configured to perform any one of the method steps of the above described embod iments.

Accord ing to another aspect, the invention relates to a motor vehicle comprising a system as defined in any one of the above described embodiments. For example, the motor vehicle may be powered by an internal combustion engine and/or by an electric motor. In particu lar, but not exclusively, the method and system may be used in a heavy motor veh icle such as a truck or a bus.

Other advantageous features as well as advantages of embodiments of the present invention will appear from the following description .

BRI EF DESCRI PTION OF THE DRAWI NGS Embodiments of the invention will in the following be further described by means of example with reference to the appended drawings, wherein

Fig . 1 shows a motor vehicle according to an embodiment of the invention ,

Fig . 2 is a flow chart illustrating the method according to an embodiment Fig . 3 is a diagram illustrating traffic acceptable constraints for use in a method according to an embodiment of the invention

Fig . 4 is a diagram illustrating driver acceptable constraints for use in a method according to an embodiment of the invention

Fig . 5 is another diagram illustrating driver acceptable constraints for use in a method accord ing to an embodiment of the invention , and Fig . 6 schematically shows an electron ic control un it of a system according to an embodiment of the invention .

DETAI LED DESCRI PTI ON OF EMBODI MENTS OF THE I NVENTION

A motor veh icle 1 in which a system 2 according to an embodiment of the invention is provided is schematically shown in fig . 1 . The motor vehicle 1 is travelling on a level road between two nearby vehicles 3, 4, i.e. behind a lead veh icle 3 and ahead of a trailing veh icle 4. A cruise control 5 is used to control vehicle speed of the motor vehicle. The system includes a forward radar equipment 6 and a rear radar equipment 7 adapted to detect and measure a current gap to the lead vehicle 3 and the trailing vehicle 4, respectively.

In a method accord ing to an embodiment of the invention , illustrated in the flow chart of fig . 2 , an input is in step S1 received from a driver of the motor vehicle 1 , defin ing a desired set speed of the motor veh icle 1 and a desired performance mode. In th is example, it is assumed that the driver selects an economic mode, adapted to minimize fuel consumption of the motor vehicle. The input from the driver defines driver acceptable constraints which may, in the absence of surrounding vehicles, be used to calculate an intended speed trajectory of the motor veh icle 1 . Thus, the driver acceptable constraints define how the vehicle speed is allowed to be varied in the absence of nearby vehicles in dependence on road gradient and road curvature. As it is in a step S2 detected by the radar equ ipment that a lead vehicle 3 and a trailing veh icle 4 are present, a current gap to each nearby vehicle is determined in a step S3, in th is case measured using the radar equ ipment 6, 7. The sizes of the gaps can be determined in terms of time or distance. Furthermore, an expected speed trajectory of each nearby vehicle 3, 4 is determined , either by estimating the speed trajectories on-board the motor veh icle 1 or by receiving the speed trajectories from the nearby veh icles 3, 4 or from a central communication node (not shown) by means of wireless commun ication .

Based on input parameters including the determined current gaps to the nearby vehicles 3, 4, the expected speed trajectories of the nearby vehicles 3, 4, and predefined minimum gap sizes specifying a minimum allowable rearward gap di to the trailing veh icle 4 and minimum allowable forward gap d 2 to the lead veh icle 3, respectively, traffic acceptable constraints are determined in a step S4. The traffic acceptable constraints are set so as to ensure that, on the upcoming road stretch , a future gap to the trailing vehicle 4 and the lead veh icle 3, respectively, will correspond to at least the minimum gap sizes. The motor veh icle's velocity will only be allowed to fluctuate such that the motor vehicle is positioned within the allowed space d a l lowed (see fig . 1 ).

Based on a current vehicle speed of the motor vehicle 1 , the driver acceptable constraints and the traffic acceptable constraints, an intended speed trajectory of the motor veh icle 1 is thereafter in a step S5 determined such that the intended speed trajectory satisfies the traffic acceptable constraints as well as the driver acceptable constraints. If not all constraints can be simultaneously satisfied , pre-defined conditions may be used to select the most suitable intended speed trajectory. For example, the traffic acceptable constraints may include a hard constraint which may never be violated relating to the minimum forward gap size d 2. The traffic acceptable constraints may also include a soft constraint relating to the min imum rearward gap size d i . This soft constraint may be defined so that it can be violated if the motor vehicle 1 is unable to maintain the set speed , e.g . on a steep uph ill road stretch , but so that it may not be violated if the motor veh icle 1 is able to maintain the minimum gap without violating the driver acceptable constraints.

The vehicle speed is thereafter controlled in a step S6, using the cruise control 5, in accordance with the intended speed trajectory. The intended speed trajectory is continuously re-calcu lated using the method according to the invention .

Simultaneously with the intended speed trajectory, associated control trajectories may also be determined , such as a torque trajectory includ ing driving and braking torques to be demanded from a powertrain of the motor vehicle 1 and/or a gear selection trajectory. The control trajectories can thereafter be used to control the powertrain such that the vehicle speed is ind irectly controlled in accordance with the intended speed trajectory.

A diagram illustrating traffic acceptable constraints is shown in fig . 3. The diagram shows distance D to be covered by the motor veh icle 1 as a function of time t, wherein the dashed line shows a constant set speed trajectory of the motor veh icle 1 , i.e. the distance that would be covered if the motor vehicle 1 would travel at a constant set speed . The u pper solid line shows the expected speed trajectory of a lead vehicle 3 travelling ahead of the motor veh icle 1 , i.e. the distance that is expected to be covered by the lead vehicle 3 including a distance d2 corresponding to the minimum forward gap size. The lower solid line shows the expected speed trajectory of a trailing vehicle 4 travelling behind the motor veh icle 1 , including a distance di corresponding to the minimum rearward gap size. The solid lines delimit an allowable space with in which the speed trajectory of the motor vehicle 1 may be allowed to vary taking the traffic acceptable constraints into account, i.e. without coming closer to the lead vehicle 3 and the trailing vehicle 4 than the minimum forward and rearward gaps, respectively.

A diagram illustrating driver acceptable constraints in terms of velocity v as a function of distance D to be covered by the motor vehicle 1 is shown in fig . 4. The dashed line shows a constant set speed trajectory of the motor vehicle 1 , i.e. the velocity as a function of distance D if travelling at a constant set speed, and the upper and lower solid lines show maximum and min imum allowed velocity variations as a function of distance D, respectively. As explained above, the solid lines delimit an allowable space with in which the velocity of the motor vehicle 1 may be varied while satisfying the driver acceptable constraints. The velocity v may e.g . be allowed to be varied with a different amount depending on road curvature and road gradient, depending on performance mode of the motor vehicle 1 . Another d iagram illustrating driver acceptable constraints, here in terms of distance deviation Δϋ as a function of distance D to be covered by the motor vehicle, is shown in fig . 5. The upper and lower lines represent driver acceptable distance deviations Δϋ from a constant set speed trajectory, i.e. by wh ich distance it is acceptable that the motor veh icle deviates from a distance D that would be travelled if travelling at a constant speed . The upper and lower lines delimit an allowable space with in which the distance travelled by the motor vehicle 1 may be varied while satisfying the driver acceptable constraints. The distance deviation Δϋ may e.g . be allowed to be varied with a different amount depending on road curvature and road gradient.

One skilled in the art will appreciate that a method for adjusting vehicle speed of a motor vehicle 1 accord ing to an embodiment of the present invention may be implemented in a computer program which , when executed in a computer, causes the computer to conduct the method . The computer program usually takes the form of a computer program product wh ich comprises a suitable d igital storage medium on which the computer program is stored . Said computer-readable digital storage medium comprises a suitable memory, e.g . ROM (read-on ly memory), PROM (programmable read-only memory), EPROM (erasable PROM), flash memory, EEPROM (electrically erasable PROM) , a hard disc unit, etc.

Fig . 6 illustrates very schematically an electron ic control un it 30 forming part of a system 2 according to an embodiment of the invention . The control un it 30 comprises an execution means 31 , such as a central processor unit (CPU) , for executing a computer program. The execution means 31 communicates with a memory 32, for example of the type RAM , through a data bus 33. The control un it 30 comprises also a non-transitory data storing medium 34, for example in the form of a Flash memory or a memory of the type ROM , PROM , EPROM or EEPROM . The execution means 31 communicates with the data storing medium 34 through the data bus 33. A computer program comprising computer program code for implementing a method according to an embodiment of the invention is stored on the data storing med ium 34.

The invention is of course not in any way restricted to the embodiments described above, but many possibilities to modifications thereof wou ld be apparent to a person with skill in the art without departing from the scope of the invention as defined in the appended claims.

Claims

CLAI MS

1 . A method for adjusting vehicle speed of a motor vehicle (1 ) in which the vehicle speed is controlled using a cruise control (5) , comprising :

- retrieving driver acceptable constraints defin ing how the vehicle speed is allowed to be automatically varied by means of the cru ise control (5),

- in response to a detection of at least one nearby vehicle (3, 4) travelling ahead of and/or behind the motor veh icle (1 ), determining a current gap to the at least one nearby veh icle (3, 4) and obtaining an expected speed trajectory of the at least one nearby vehicle (3, 4),

- based on the determined current gap, the expected speed trajectory of the at least one nearby veh icle (3, 4), and at least one predefined minimum gap size (di , d2) , determining traffic acceptable constraints set so as to ensure that a future gap to the at least one nearby vehicle (3, 4) will correspond to at least the minimum gap size (di , d2) ,

- based on a current vehicle speed , the driver acceptable constraints and the traffic acceptable constraints, determining an intended speed trajectory of the motor veh icle (1 ) satisfying at least one of said constraints,

- controlling a powertrain of the motor vehicle (1 ) such that the vehicle speed is adjusted based on the intended speed trajectory.

2. The method accord ing to claim 1 , wherein , if the at least one nearby vehicle (3, 4) includes a lead vehicle (3) travelling ahead of the motor veh icle (1 ), the traffic acceptable constraints include a hard constraint on a minimum forward gap (d2) to the lead veh icle (3), which hard constraint must be satisfied by the intended speed trajectory.

3. The method accord ing to claim 1 , wherein , if the at least one nearby vehicle (3, 4) includes a trailing veh icle (4) travelling beh ind the motor veh icle (1 ) , the traffic acceptable constraints include a soft constraint on a minimum rearward gap (di ) to the trailing veh icle (4) , which soft constraint is allowed to be selectively satisfied by the intended speed trajectory.

4. The method according to any one of the preceding claims, wherein the driver acceptable constraints include a maximum allowable distance deviation (Δϋ) from a desired constant set speed trajectory of the motor vehicle (1 ).

5. The method according to any one of the preceding claims, wherein the driver acceptable constraints include a position dependent maximum and/or minimum allowable vehicle speed (v) .

6. The method according to any one of the preceding claims, wherein the determination of the intended speed trajectory comprises a rule based method and/or an optimization based method .

7. The method according to claim 6, wherein the determination of the intended speed trajectory includes:

- calculating a number of possible speed trajectories according to predefined rules, - based on said constraints, setting a status of the possible speed trajectories to allowable or non-allowable,

- based on predefined selection criteria, selecting one of the allowable speed trajectories as the intended speed trajectory.

8. The method according to claim 7, wherein the predefined selection criteria are set so as to select the most fuel efficient allowable speed trajectory.

9. The method according to any one of the preceding claims, further comprising determining at least one intended control trajectory associated with the intended speed trajectory, wherein the at least one intended control trajectory is used to control the powertrain of the motor vehicle (1 ).

10. The method accord ing to any one of the preceding claims, wherein determining a current gap to the nearby vehicle (3, 4) comprises measuring the current gap.

1 1 . The method according to any one of the preceding claims, wherein the at least one min imum gap size (di , d2) is determined based on at least one of a vehicle speed , a vehicle acceleration , brake action , tyre quality, road conditions, weather, and traffic conditions.

12. A computer program comprising instructions which , when the computer program is executed by a computer, cause the computer to carry out the method according to any one of the preceding claims.

13. A computer-readable med ium comprising instructions which , when executed by a computer, cause the computer to carry out the method according to any one of claims 1 -1 1 .

14. A system (2) for adjusting vehicle speed of a motor veh icle (1 ) in wh ich the veh icle speed is controlled using a cruise control (5), comprising a control unit (30) configured to:

- retrieve driver acceptable constraints defining how the vehicle speed is allowed to be automatically varied by means of the cru ise control (5),

- in response to a detection of at least one nearby veh icle (3, 4) travelling ahead of and/or behind the motor veh icle (1 ), determine a current gap to the at least one nearby vehicle (3, 4) and obtain an expected speed trajectory of the at least one nearby vehicle (3, 4),

- based on the determined current gap, the expected speed trajectory of the at least one nearby vehicle (3, 4), and at least one predefined minimum gap size (d i , d 2) , determine traffic acceptable constraints set so as to ensure that a future gap to the at least one nearby vehicle (3, 4) will correspond to at least the minimum gap size (di , d2) ,

- based on a current vehicle speed , the driver acceptable constraints and the traffic acceptable constraints, determine an intended speed trajectory of the motor vehicle ( 1 ) satisfying at least one of said constraints,

- control a powertrain of the motor vehicle (1 ) such that the vehicle speed is adjusted based on the intended speed trajectory.

15. A motor vehicle (1) comprising a system (2) according to claim 14.