Rodman, J.

By motions to nonsuit defendants challenge the right of plaintiffs to recover. The reasons assigned are: (1) Defendant’s vehicle was “an authorized emergency vehicle” on an emergency errand, and as such, given by statute and ordinance priority in the right to use the intersection and the right to travel at a speed made unlawful as to other vehicles. They merely exercised the rights accorded the ambulance, and negligence cannot be predicated on the exercise of legal rights. (2) Williams, operator of the Chevrolet, was contributorily negligent in (a) failing to yield the right of way to defendant’s vehicle and (b) in failing, to maintain a reasonable and proper lookout for emergency and other vehicles on the intersecting highway. : ■ ■ .

*528 This collision occurred in a municipality. Hence the provisions of G.S. 20-158(c), relating to traffic lights outside of towns, has no application. No ordinance of Morganton declaring the consequences of a failure to heed the light was offered in evidence. The operators of the motor vehicles were, therefore, to interpret the signals and give that obedience thereto which a reasonably prudent operator would give. Wilson v. Kennedy, 248 N.C. 74.

The use of these lights is too general and well-known to raise any doubt as to meaning of each color and what is expected of an operator when confronted with a red light. It tells him to stop. Defendants recognize this meaning and ordinary application. They concede for the purpose of this appeal Williams had the green light and Miller, the red or stop light. But they say the red did not tell Miller to stop because (a) the State statute, G.S. 20-156, gave Miller the right of way, and (b) Morganton’s ordinance declared authorized emergency vehicles may “proceed past red or stop signal or stop sign but only after slowing down as may be necessary for operation.”

These laws do accord a privilege, but to exercise the privilege one must establish that he belongs to the privileged class. As said in 35 C.J.S. 185: “As a general rule, in an action or proceeding to enforce or establish an exemption right the burden is on him who seeks to enforce or establish it.” Applications of the rule are illustrated in Sabine v. Gill, 229 N.C. 599, 51 S.E. 2d 1; Henderson v. Gill, 229 N.C. 313, 49 S.E. 2d 754; S. v. Kelly, 186 N.C. 365, 119 S.E. 755; S. v. Simmons, 143 N.C. 613; S v Hayne, 88 N.C. 625; S. v. Hogg, 6 N.C. 319; Williams v. Branson, 5 N.C. 417; Oakley v. Alleghany County, 193 A 316.

For the purpose of this appeal we may, as the parties and trial court apparently did, treat “official business,” when applied to private automobiles, as meaning a trip made when the operator of the vehicle bona fide believes an emergency exists .which requires expeditious movement. If the driver in fact has such belief and meets the statutory test by giving warning, he is accorded the necessary privilege. The audible sound required by the statute is one heard and understood or which should have been heard and its meaning understood by a reasonably prudent operator called upon to yield the right of way. Funeral Service v. Coach Lines, 248 N.C. 146.

Notwithstanding the unequivocal evidence from witnesses for defendants that they heard the siren when the ambulance began its journey and continued to hear it until the moment of the impact, the equally unequivocal testimony of the occupants of the Chevrolet and another witness just a few feet from the intersection that they did not hear the siren until the ambulance was within a few feet of the intersection is some evidence that the siren was not in fact sounded in time *529 to provide a warning to the plaintiffs. Carruthers v. R.R., 218 N.C. 49, 9 S.E. 2d 498; Johnson v. R.R., 205 N.C. 127, 170 S.E. 120; Edwards v. R.R., 129 N.C. 78.

With this discrepancy in the evidence, the court could not, as a matter of law, hold that Miller had complied with the terms of the statute and was entitled to the right of way.

It is true, as defendants say, that the ordinance of Morganton which permits ambulances to “proceed past red or stop signals” does not require the siren to be sounded, but it does limit their right to proceed “only after slowing down as may be necessary for operation.” This necessarily means, we think, that the special privilege can only be exercised when the ambulance can proceed with safety to others who have a legal invitation to use the intersection. To give it any other interpretation would change an ordinance intended to facilitate the safe movement of vehicles across intersecting highways into a trap for those invited to enter.

With the burden of proof on defendants to establish they belonged in the privileged class described in the statute and ordinance, the court could not, as a matter of law, hold that they had the prior right to use the intersection. If they did not have such right, plaintiffs were not, as a matter of law, negligent in accepting the invitation extended to them by the green light.

The assertion that the evidence establishes without contradiction that Williams failed to exercise the vigilance of a reasonably prudent driver is also raised by exceptions to the charge and is discussed in that connection.

The motion to nonsuit was properly overruled and the issue of contributory negligence was properly submitted to the jury.

Assignments of error 8, 9, 10, 11, 12, 13, 14, and 15 question the accuracy and sufficiency of the charge as it relates to the duty of an operator to look for the movement of other vehicles at intersections and the duty of an operator of an ordinary vehicle to yield the right of way to emergency vehicles.

The court, in charging the jury, gave as the basic rules applicable to the rights ahd duties of plaintiff Williams, the law as stated in Cox v. Freight Lines, 236 N.C. 72, 72 S.E. 2d 25, and Hyder v. Battery Co., 242 N.C. 553, 89 S.E. 2d 124. The jury was told that notwithstanding a green traffic light faced the operator of a motor vehicle, he could not go forward blindly, but was required to use ordinary care, to maintain a proper lookout, to keep his vehicle under reasonable control, and to drive his vehicle at a speed reasonable and prudent under existing conditions. Dealing specifically with the duty to yield the right of way to emergency vehicles, the court charged: “Regardless of *530 whether the plaintiff actually heard the siren, if you find from the evidence that the siren was audibly sounded and that the plaintiff was within range where he could have heard the siren had he been listening, then the plaintiff will be deemed to have heard that which he should have heard and that which a reasonably prudent person exercising due care would have heard.

“The operator of an authorized emergency vehicle, being the defendant Miller in this case, while on an emergency call, has the right to proceed upon the assumption that when the required signal by siren is given, that other users of the highway will yield the right of way.”

We think the court correctly measured the rights of an operator of an emergency vehicle and the duty of the driver of an ordinary vehicle to respect that right.

Williams testified that when three or four or five car lengths from the intersection, he observed the traffic light was green. He continued to observe it until he entered the intersection and so long as he could see the light, it remained green. On cross-examination defendants sought to show that Williams’ attention was on the traffic light to the exclusion of traffic on the streets. In response to questions asked, he said that he looked for approaching traffic. Repeated questions led him to estimate the places at which he looked or attempted to look at four to eight car lengths from the intersection. Defendants point to the fact that vision down the intersecting street was obstructed until, the driver reached a point estimated at 30 to 50 feet from the intersection. They contend that to look eight oar lengths — 100 feet or more, they say— from the intersection would avail nothing. Such attempt to look would, as a matter of law, fail to meet the standard of the prudent man and the court erred in not so informing the jury instead of leaving it to the jury to measure defendants’ conduct under the rales which it bad given.

The rale applicable to the duty of a motorist confronted with a green light is not to be measured by the duty of one traversing a railroad crossing. A railroad crossing is itself notice of danger. Coleman v. R.R., 153 N.C. 322; 69 S.E. 251; Quinn v. R.R., 213 N.C. 48, 195 S.E. 85. The traveler knows trains do not normally stop at highway crossings, and from the nature of the operation are not expected to stop. Reasonable prudence dictates the traveler should take precaution to see that he can cross in safety before entering the crossing.

Automobiles, unlike trains, may be stopped in comparatively short distances. They are not confined to a single line of movement. Of course the motorist must, as the court charged, be vigilant. He must remain alert and see and-heed those things which a prudent driver *531 would see and guard against, but he is not required to anticipate negligence on the part of other drivers. Jackson v. McCoury, 247 N.C. 502; Simmons v. Rogers, 247 N.C. 340; Williamson v. Randall, 248 N.C. 20; Hyder v. Battery Co., supra.

We think the court’s charge correct, when read as a whole, and no error was committed in permitting the jury to determine whether on all the evidence Williams was reasonably vigilant in the operation of his vehicle.

We interpret the ordinance of Morganton authorizing the chief of police to designate ambulances which may be used as emergency vehicles as empowering him to license specific vehicles for that purpose and not to authorize him to grant blanket authority to some person to operate any vehicle as an ambulance in an emergency. Sound reason would seem to exist for the interpretation we place on it. Presumably he would not authorize or license a particular vehicle that was not properly and adequately equipped both to handle patients and to warn other operators of its approach when used in an emergency. All of the ordinances are not in the record, and we do not know what standards were prescribed in order to obtain authorization. The constitutionality of the ordinance is not here challenged.

The evidence which defendants offered with respect to the authority to operate emergency vehicles by Sossoman’s Funeral Home does not appear to be directed to this specific vehicle but to vehicles in general.

But conceding that due authority had been given to operate this particular vehicle in emergencies, the ordinance, as noted with respect to the motion to nonsuit, does not, as we read it, do more than permit its operator to disregard the red light when he can do so with safety. The charge dealing with .the duty of the drivers to exercise due care in the operation of. their vehicles was, we think, sufficient to cover this phase of the case.

Our examination of the assignments discloses no prejudicial error.

No error.

JohnsoN, J., took no part in the consideration or decision of this case.