Speed difference indicator on head up display

1. A head up display (HUD) system, comprising:

a difference module configured to determine a speed difference based on a difference between (a) a present speed of a vehicle and (b) a target speed of the vehicle;

a light source configured to, via a windshield of the vehicle, generate a virtual display that is visible within a passenger cabin of the vehicle;

a perception module configured to identify boundaries of a present lane of the vehicle, the boundaries of the present lane decreasing in width from a point of view of a driver; and

a display control module configured to:

control the light source to include a visual indicator of the speed difference in the virtual display behind an object located directly in front of the vehicle such that the visual indicator of the speed difference does not overlap the object;

increase a length of the visual indicator as a magnitude of the speed difference increases;

decrease the length of the visual indicator as the magnitude of the speed difference decreases; and

display the visual indicator between the boundaries of the present lane and such that a width of the visual indicator decreases from the point of view of the driver with the width of the boundaries of the present lane.

2. The HUD system of claim 1 wherein the difference module is configured to set the speed difference based on the present speed of the vehicle minus the target speed of the vehicle.

3. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on a cruise control speed.

4. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on a speed limit of a road at a location of the vehicle.

5. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on a speed determined based on fuel efficiency of the vehicle.

6. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on weather at a location of the vehicle.

7. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on a road condition at a location of the vehicle.

8. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on an estimated time of arrival of the vehicle to a destination location.

9. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on a speed for a curve in a road.

10. The HUD system of claim 1 wherein the display control module is configured to adjust at least one attribute of the visual indicator as the speed difference changes.

11. The HUD system of claim 10 wherein the display control module is configured to:

further display a reference;

display the reference in a first position further from the vehicle than the reference when the speed difference is positive; and

display the reference in a second position closer to the vehicle than the reference when the speed difference is negative,

wherein the first position is different than the second position.

12. The HUD system of claim 10 wherein the display control module is configured to:

display the visual indicator in a first color when the speed difference is positive; and

display the visual indicator in a second color when the speed difference is negative,

wherein the first color is different than the second color.

13. The HUD system of claim 10 wherein the display control module is configured to:

decrease a transparency of the visual indicator as a magnitude of the speed difference increases; and

increase the transparency of the visual indicator as the magnitude of the speed difference decreases.

14. The HUD system of claim 1 wherein the visual indicator includes a magnitude of the speed difference and an indicator of whether the speed difference is positive or negative.

15. The HUD system of claim 1 further comprising:

an eye location module configured to determine a location of eyes of the driver of the vehicle,

wherein the display control module is configured to move the visual indicator based on the location of the eyes of the driver; and

a disabling module configured to selectively disable the display of the visual indicator when a gaze the driver is not toward a road in front of the vehicle.

16. The HUD system of claim 1 further comprising a target module configured to set the target speed of the vehicle based on at least two of:

a cruise control speed;

a speed limit of a road at a location of the vehicle;

a speed determined based on efficiency of the vehicle;

weather at a location of the vehicle;

a road condition at a location of the vehicle;

an estimated time of arrival of the vehicle to a destination location; and

a speed for a curve in a road.

17. A head up display (HUD) method, comprising:

determining a speed difference based on a difference between (a) a present speed of a vehicle and (b) a target speed of the vehicle;

via a windshield of the vehicle, by a light source, generating a virtual display that is visible within a passenger cabin of the vehicle;

identifying boundaries of a present lane of the vehicle, the boundaries of the lane decreasing in width from a point of view of a driver;

controlling the light source to include a visual indicator of the speed difference in the virtual display behind an object located directly in front of the vehicle such that the visual indicator of the speed difference does not overlap the object;

increasing a length of the visual indicator as a magnitude of the speed difference increases;

decreasing the length of the visual indicator as the magnitude of the speed difference decreases; and

displaying the visual indicator between the boundaries of the present lane and such that a width of the visual indicator decreases from the point of view of the driver with the width of the boundaries of the present lane.